1.5 Volt LED Flashers

The LED flasher circuits below operate on a single 1.5 volt battery. The circuit on the upper right uses the popular LM3909 LED flasher IC and requires only a timing capacitor and LED.

The top left circuit, designed by Andre De-Guerin illustrates using a 100uF capacitor to double the battery voltage to obtain 3 volts for the LED. Two sections of a 74HC04 hex inverter are used as a squarewave oscillator that establishes the flash rate while a third section is used as a buffer that charges the capacitor in series with a 470 ohm resistor while the buffer output is at +1.5 volts. When the buffer output switches to ground (zero volts) the charged capacitor is placed in series with the LED and the battery which supplies enough voltage to illuminate the LED. The LED current is approximately 3 mA, so a high brightness LED is recommended.

In the other two circuits, the same voltage doubling principle is used with the addition of a transistor to allow the capacitor to discharge faster and supply a greater current (about 40 mA peak). A larger capacitor (1000uF) in series with a 33 ohm resistor would increase the flash duration to about 50mS. The discrete 3 transistor circuit at the lower right would need a resistor (about 5K) in series with the 1uF capacitor to widen the pulse width.
Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

AC Line powered LEDs

The circuit below illustrats powering a LED (or two) from the 120 volt AC line using a capacitor to drop the voltage and a small resistor to limit the inrush current. Since the capacitor must pass current in both directions, a small diode is connected in parallel with the LED to provide a path for the negative half cycle and also to limit the reverse voltage across the LED. A second LED with the polarity reversed may be subsituted for the diode, or a tri-color LED could be used which would appear orange with alternating current. The circuit is fairly efficient and draws only about a half watt from the line. The resistor value (1K / half watt) was chosen to limit the worst case inrush current to about 150 mA which will drop to less than 30 mA in a millisecond as the capacitor charges. This appears be a safe value, I have switched the circuit on and off many times without damage to the LED. The 0.47 uF capacitor has a reactance of 5600 ohms at 60 cycles so the LED current is about 20 mA half wave, or 10 mA average. A larger capacitor will increase the current and a smaller one will reduce it. The capacitor must be a non-polarized type with a voltage rating of 200 volts or more.

The lower circuit is an example of obtaining a low regulated voltage from the AC line. The zener diode serves as a regulator and also provides a path for the negative half cycle current when it conducts in the forward direction. In this example the output voltage is about 5 volts and will provide over 30 milliamps with about 300 millivolts of ripple. Use caution when operating any circuits connected directly to the AC line.
Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info


LED Traffic Lights

The LED traffic Light circuit controls 6 LEDs (red, yellow and green) for both north/south directions and east/west directions. The timing sequence is generated using a CMOS 4017 decade counter and a 555 timer. Counter outputs 1 through 4 are wire ORed using 4 diodes so that the (Red - North/South) and (Green - East/West) LEDs will be on during the first four counts. The fifth count (pin 10) illuminates (Yellow - East/West) and (Red - North/South). Counts 6 through 9 are also wire ORed using diodes to control (Red - East/West) and (Green - North/South). Count 10 (pin 11) controls (Red - East/West) and (Yellow - North/South). The time period for the red and green lamps will be 4 times longer than for the yellow and the complete cycle time can be adjusted with the 47K resistor. The eight 1N914 diodes could be subsituted with a dual 4 input OR gate (CD4072).
Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info


40 LED Bicycle Light

The 555 circuit below is a flashing bicycle light powered with four C,D or AA cells (6 volts). Two sets of 20 LEDs will alternately flash at approximately 4.7 cycles per second using RC values shown (4.7K for R1, 150K for R2 and a 1uF capacitor). Time intervals for the two lamps are about 107 milliseconds (T1, upper LEDs) and 104 milliseconds (T2 lower LEDs). Two transistors are used to provide additional current beyond the 200 mA limit of the 555 timer. A single LED is placed in series with the base of the PNP transistor so that the lower 20 LEDs turn off when the 555 output goes high during the T1 time interval. The high output level of the 555 timer is 1.7 volts less than the supply voltage. Adding the LED increases the forward voltage required for the PNP transistor to about 2.7 volts so that the 1.7 volt difference from supply to the output is insufficient to turn on the transistor. Each LED is supplied with about 20 mA of current for a total of 220 mA. The circuit should work with additional LEDs up to about 40 for each group, or 81 total. The circuit will also work with fewer LEDs so it could be assembled and tested with just 5 LEDs (two groups of two plus one) before adding the others.
Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

16 Stage Bi-Directional LED Sequencer

The bi-directional sequencer uses a 4 bit binary up/down counter (CD4516) and two "1 of 8 line decoders" (74HC138 or 74HCT138) to generate the popular "Night Rider" display. A Schmitt Trigger oscillator provides the clock signal for the counter and the rate can be adjusted with the 500K pot. Two additional Schmitt Trigger inverters are used as a SET/RESET latch to control the counting direction (up or down). Be sure to use the 74HC14 and not the 74HCT14, the 74HCT14 may not work due to the low TTL input trigger level. When the highest count is reached (1111) the low output at pin 7 sets the latch so that the UP/DOWN input to the counter goes low and causes the counter to begin decrementing. When the lowest count is reached (0000) the latch is reset (high) so that the counter will begin incrementing on the next rising clock edge. The three lowest counter bits (Q0, Q1, Q2) are connected to both decoders in parallel and the highest bit Q3 is used to select the appropriate decoder. The circuit can be used to drive 12 volt/25 watt lamps with the addition of two transistors per lamp as shown below in the section below titled "Interfacing 5 volt CMOS to 12 volt loads".
Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

Expandable 16 Stage LED Sequencer

The circuit below uses a hex Schmitt Trigger inverter (74HC14) and two 8 bit Serial-In/Parallel-Out shift registers (74HCT164 or 74HC164) to sequence 16 LEDs. The circuit can be expanded to greater lengths by cascading additional shift registers and connecting the 8th output (pin 13) to the data input (pin 1) of the succeeding stage. A Schmitt trigger oscillator (74HC14 pin 1 and 2) produces the clock signal for the shift registers, the rate being approximately 1/RC. Two additional Schmitt Trigger stages are used to reset and load the registers when power is turned on. Timing is not critical, however the output at pin 8 of the Schmitt Trigger must remain high during the first LOW to HIGH clock transition at pin 8 of the registers, and must return low before the second rising edge to load a single bit. If the clock rate is increased, the length of the signal at pin 9 of the Schmitt Trigger should be reduced proportionally to avoid loading more than one bit. The HCT devices will normally provide about 4 mA (source or sink) from each output but can supply greater currents (possibly 25 mA) if only one output is loaded. The common 150 ohm resistor restricts the current below 25 mA using a 6 volt power source. If the circuit is operated with two or more LEDs on at the same time, resistors may be needed in series with each LED to avoid exceeding the maximum total output current for each IC of 25 mA. For greater brightness, individual buffer transistors can be used as shown in the 10 stage LED sequencer on this same page.
Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

10 Channel LED Sequencer

The 4017 is a CMOS decade counter with 10 decoded outputs. Inputs include a CLOCK (pin14), a RESET  (PIN15), and a CLOCK INHIBIT (pin 13). The clock input drives an internal schmitt trigger circuit for pulse shaping and allows for unlimited clock rise and fall times. The counter is advanced one count at the rising edge of the clock signal if the CLOCK INHIBIT line is low. A high RESET signal resets the counter to the zero count. The circuit may be configured for counts less than 10 by connecting RESET to an output pin one above the desired count. Thus, a five channel sequencer could be made by connecting pin 15 to pin 1. A CARRY-OUT signal (pin 12) may be used to clock subsequent stages in a multi-device counting chain (ones, tens, hundred, etc). Small signal NPN transistors are used to increase the output current for the LEDs to about 20ma which is set by the common 120 ohm resistor. Other NPN transistors may be substituted for the 3904. The 555 timer generates the clock signal, the frequency being determined by the 1uF capacitor and 47K resistor which is approximately = 1.44 / 2RC = 15 Hz.
Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info


Two Transistor LED Flasher

This circuit will flash a bright red LED (5000 mcd) as an attention getting device or fake car alarm. Component values are not critical and other transistors may be used. Flash duration is determined by R2 and C1 and is approximately 3 time constants (3*R2*C1). Brightness is controlled by R3 wich limits the LED current to about 20 mA for values listed. R1 provides bias for the transistors which should be low enough not to saturate Q2 with the capacitor disconnected. If the circuit does not oscillate, R1 may be too low or R2 may be too high. D1 allows for higher duty cycle operation and limits the reverse voltage at the base of Q1 to -0.7 V. D1 may be omitted for low voltage (3-9) and low duty cycle operation. Most parts available at Radio Shack.
Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info





Volts R1 R2 R3 C1 Approx. Flash Rate
12 10 Mohm 22 Kohm 470 ohm 0.47 uF 140 per minute
12 10 Mohm 10 Kohm 470 ohm 1 uF 60 per minute
9 6.8 Mohm 1 Kohm 390 ohm 6.8 uF 15 per minute
6 3.3 Mohm 10 Kohm 220 ohm 1 uF 80 per minute
3 1.5 Mohm 10 Kohm 51 ohm 1 uF 120 per minute
3 3.3 Mohm 47 Kohm 51 ohm 0.47 uF 140 per minute


Fading Red Eyes

This circuit can be used to slowly illuminate and fade a pair of red LEDs. A linear 3 volt p-p ramping wavform is generated at pin 1 of the IC and buffered with an emitter follower transistor stage. The 22uF capacitor and 47K resistor connected to pin 2 establish the frequency which is about 0.5 Hz.
Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info





28 LED Clock Timer

Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

This is a programmable clock timer circuit that uses individual LEDs to indicate hours and minutes. 12 LEDs can be arranged in a circle to represent the 12 hours of a clock face and an additional 12 LEDs can be arranged in an outer circle to indicate 5 minute intervals within the hour. 4 additional LEDs are used to indicate 1 to 4 minutes of time within each 5 minute interval.

The circuit is powered from a small 12.6 volt center tapped line transformer and the 60 cycle line frequency is used for the time base. The transformer is connected in a full wave, center tapped configuration which produces about 8.5 volts unregulated DC. A 47 ohm resistor and 5.1 volt, 1 watt zener regulate the supply for the 74HCT circuits.

A 14 stage 74HCT4020 binary counter and two NAND gates are used to divide the line frequency by 3600 producing a one minute pulse which is used to reset the counter and advance the 4017 decade counter. The decade counter counts the minutes from 0 to 4 and resets on the fifth count or every 5 minutes which advances one section of a dual 4 bit binary counter (74HCT393). The 4 bits of this counter are then decoded into one of 12 outputs by two 74HCT138 (3 line to 8 line) decoder circuits. The most significant bit is used in conjunction with an inverter to select the appropriate decoder. During the first eight counts, the low state of the MSB is inverted to supply a high level to enable the decoder that drives the first 8 LEDs. During counts 9 to 12, the MSB will be high and will select the decoder that drives the remaining 4 LEDs while disabling the other decoder. The decoded outputs are low when selected and the 12 LEDs are connected common anode with a 330 ohm current limiting resistor to the +5 volt supply. The 5th output of the second decoder (pin 11) is used to reset the binary counter so that it counts to 11 and then resets to zero on the 12th count. A high reset level is required for the 393 counters, so the low output from the last decoder stage (pin 11) is inverted with one section of a 74HCT14 hex Schmitt trigger inverter circuit. A 10K resistor and 0.1uF cap are used to extend the reset time, ensuring the counter receives a reset signal which is much longer than the minimum time required. The reset signal is also connected to the clock input (pin 13) of the second 4 bit counter (1/2 74HCT393) which advances the hour LEDs and resets on the 12th hour in a similar manner.

Setting the correct time is accomplished with two manual push buttons which feed the Q4 stage (pin 7) of the 4020 counter to the minute and hour reset circuits which advance the counters at 3.75 counts per second. A slower rate can be obtained by using the Q5 or Q6 stages. For test purposes, you can use Q1 (pin 9) which will advance the minutes at 30 per second.

The time interval circuit (shown below the clock) consists of a SET/RESET flipflop made from the two remaining NAND gates (74HCT00). The desired time interval is programmed by connecting the anodes of the six diodes labeled start, stop and AM/PM to the appropriate decoder outputs. For example, to turn the relay on at 7:05AM and turn it off at 8:05AM, you would connect one of the diodes from the start section to the cathode of the LED that represents 7 hours, the second diode to the LED cathode that represents 5 minutes and the third diode to the AM line of the CD4013. The stop time is programmed in the same manner. Two additional push buttons are used to manually open and close the relay. The low start and stop signals at the common cathode connections are capacitively coupled to the NAND gates so that the manual push buttons can override the 5 minute time duration. That way, you can immediately reset the relay without waiting 5 minutes for the start signal to go away.

The two power supply rectifier diodes are 1N400X variety and the switching diodes are 1N914 or 4148s but any general purpose diodes can be used. 0.1 uF caps (not shown on schematic) may be needed near the power pins of each IC. All parts should be available from Radio Shack with the exception of the 74HCT4017 decade counter which I didn't see listed. You can use either 74HC or 74HCT parts, the only difference between the two is that the input switching levels of the HCT devices are compatible with worst case TTL logic outputs. The HC device inputs are set at 50% of Vcc, so they may not work when driven from marginal TTL logic outputs. You can use a regular 4017 in place of the 74HCT4017 but the output current will much lower (less than 1 mA) and 4 additional transistors will be required to drive the LEDs. Without the buffer transistors, you can use a 10K resistor in place of the 330 and the LEDs will be visible, but very dim. Using the 4017 to drive LEDs with transistor buffers is shown in the "10 Channel LED Sequencer" at the top of this page.

72 LED Clock

Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

In the circuit below, 60 individual LEDs are used to indicate the minutes of a clock and 12 LEDs indicate hours. The power supply and time base circuitry is the same as described in the 28 LED clock circuit above. The minutes section of the clock is comprised of eight 74HCT164 shift registers cascaded so that a single bit can be recirculated through the 60 stages indicating the appropriate minute of the hour. Only two of the minutes shift registers are shown connected to 16 LEDs. Pin 13 of each register connects to pin 1 of the next for 7 registers. Pin 6 of the 8th register should connect back to pin 1 of the first register using the 47K resistor. Pins 2,9,8, 14 and 7 of all 8 minutes registers (74HC164) should be connected in parallel (pin 8 to pin 8, pin 9 to pin 9, etc.). The hours section contains two 8 bit shift registers and works the same way as the minutes to display 1 of 12 hours. Pin 9 of all 74HCT164s (hours and minutes) should be connected together. For 50 Hertz operation, the time base section of the circuit can be modified as shown in the lower drawing labeled "50 Hertz LED Clock Time Base". You will need an extra IC (74HC30) to do this since it requires decoding 7 bits of the counter instead of 4. The two dual input NAND gates (1/2 74HC00) that are not used in the 50 Hertz modification should have their inputs connected to ground.

When power is applied, a single "1" bit is loaded into the first stage of both the minutes and hours registers. To accomplish this, a momentary low reset signal is sent to all the registers (at pin 9) and also a NAND gate to lock out any clock transitions at pin 8 of the minutes registers. At the same time, a high level is applied to the data input lines of both minutes and hours registers at pin 1. A single positive going clock pulse (at pin 8) is generated at the end of the reset signal which loads a high level into the first stage of the minutes register. The rising edge of first stage output at pin 3 advances the hours (at pin 8) and a single bit is also loaded into the hours register. Power should remain off for about 3 seconds or more before being re-applied to allow the filter and timing capacitors to discharge. A 1K bleeder resistor is used across the 1000uF filter capacitor to discharge it in about 3 seconds. The timing diagram illustrates the power-on sequence where T1 is the time power is applied and beginning of the reset signal, T2 is the end of the reset signal, T3 is the clock signal to move a high level at pin 1 into the first register, T4 is the end of the data signal. The time delay from T2 to T3 is exaggerated in the drawing and is actually a very short time of just the propagation delay through the inverter and gate.

Two momentary push buttons can be used to set the correct time. The button labeled "M" will increment the minutes slowly and the one labled "H" much faster so that the hours increment slowly. The hours should be set first, followed by minutes.

50 Hertz LED Clock Timebase

Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

Infrared Remote Control Tester

This is a fairly easy circuit that can be used to test TV and VCR remote controls. The infrared detector module (GP1U52X) (Radio Shack 276-137) produces a 5 volt TTL pulse train corresponding to the digital code of the particular remote control key pressed. In the lower circuit, the module output is normally low with no signal received and becomes a positive going pulse train when a signal is present. Other detector modules are available that have an inverted output as shown in the upper drawing which is the type I used, but I don't have the part number, I believe it was removed from a VCR. The pulse sequence represents the digital code of the particular key pressed along with possible manufacturer information. As the pulse train occurs, the 4.7uF capacitor is charged to about 3 volts and the capacitor voltage minus a diode drop appears across the 470 ohm resistor yielding a collector current from the 2N3904 or 2N3906 of about 5 milliamps. The collector current of the first stage flows into the base of the output transistor (MJE34 or 2N3053) which delivers around 250 mA into the indicator lamps. When the pulse train ends, the capacitor slowly discharges through the base of the first stage transistor allowing the Xmas tree lights to remain on for a about 1 second. The little Xmas lamps will operate over a wide voltage range, so you can use bulbs from almost any string, but bulbs from shorter strings (35 or less) will probably last longer operated at 5 volts.

The circuit can be powered from a small 9-12 volt DC, 250 mA or greater wall transformer. It may also need an additional 1000 uF filter capacitor across the DC output if the wall transformer does not have a built in capacitor. For use with a 9 volt battery, the incandescent lamps can be replaced with a regular LED and 680 ohm resistor and the output transistors can be replaced with small signal transistors (2N3904 or 2N3906). The total current drain will be about 25 mA with the LED lit, and 15 mA standby when the LED is off.
Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

Battery Equal Charge Indicator

The circuit below illuminates an LED to indicate unequal charges between two 12 volt lead batteries. It can be used to verify that two batteries are connected in parallel or isolated since the LED will be off when the voltages are equal within a tollerance, or on if the voltage difference is greater than 100 millivolts. Three comparators and three voltage dividers are used to determine battery conditions. The upper left comparator (+) input at pin 5 is set to about 10 volts with battery #1 at 12 volts. The negative input (pin 4) is set to a slightly lower voltage by adding an additional 240 ohms to the voltage divider so that the output of the comparator will be positive when both battery voltages are equal and negative if battery 2 rises above battery 1 by 100 millivolts or more. The voltage at pin 5 is used as a reference for the lower comparator and the negative input of the lower comparator is set to a lower voltage with the addition of 510 ohms, so that the output will also be positive when the battery voltages are equal and negative when battery #1 is greater than #2 by 100 millivolts or more. The two comparator outputs are both connected to the positive input of the third comparator at pin 9 so that the LED will illuminate when either condition exists,
(Battery #1 > Battery #2) OR (Battery #2 > Battery #1).
Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

Astable Multivibrator

This is an astable multivibrator circuit to alternately flash two LEDs. The R and C values determine the frequency and the 470ohm collector load resistor set the current to about 20mA when the circuit is operated at 12 V. Frequency is about 1 cycle per second using 22uF capacitors and 47K resistors. Smaller R or C values will increase frequency. The LEDs may also be vired in series with the collector resistors and other transistors can be used. Reverse the supply voltage connections and LED connections if PNP type transistors are used.
Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info



LED Photo Sensor.

Here's a circuit that takes advantage of the photo-voltaic voltage of an ordinary LED. The LED voltage is buffered by a junction FET transistor and then applied to the inverting input of an op-amp with a gain of about 20. This produces a change of about 5 volts at the output from darkness to bright light. The 100K potentiometer can be set so that the output is around 7 volts in darkness and falls to about 2 volts in bright light.

Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info


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