A fool-proof VOX (voice operated transmission circuit ). 

We need VOX in a lot of situation. They are common built in features for high rank radios and some sport style hand held transceivers. Home brew add on modules are also easy to construct. Enclosed a circuit which had been used in many other application before. The configuration described here is optimized for IPHONE/radio operation.

Circuit description and operating theory

This circuit uses only two integrated circuits (RC4558 and MC14538) and two NPN transistors. Signal from computer line out (or an attenuated speaker out ) is amplified by 2N3904 and section A of RC4558. Gain of the stage can be controlled by increasing the 100K resistor between pin 1/2 of the op amp. One important note is that the ref. voltage at pin 3 of the op amp is not set at Vcc/2. It is set at about Vcc/10. This created a situation that once signal level from computer audio can push the voltage at pin 2 of op-amp A over Vcc/10, the output of the op amp will turns full Vcc.

The output will be turned off once the input voltage reduced. The output of the op-amp is thus a pulsed like signal, appeared once audio is present at the input. We need a delayed and extended PTT action with the appearance of audio. This pulsed output is used to trigger a mono-stable programmed to sense a positive edge growing pulse. The mono-stable is programmed to generate a pulse of fixed duration when-ever rise edges are detected in the feed-in signal. If a second rise edge is sensed before the pulse is completely sent, the pulse timer will reset and another full duration output pulse will be generated without a break. This is called a re-trigger flip flop.

The more sensitive the audio gain, the higher the chance that the circuit can be triggered by error pick up. VOX should cause key up activity by audio locally (speech sound from the local operator). It should not be confused by sound generated from the receive speaker. But unfortunately, audio pick up by the microphone is always un-avoidable if a speaker output is used for the receiver. That is why practical VOX circuit always comes with an anti-vox circuit. The anti-vox circuit reduces gain of the vox path when audio is detected at the speaker. This will suppress the chances of vox being triggered when the station is listening to an incoming audio. That also implies one issue - the station cannot transmit when audio is present at the receive path.


Line up procedures

The circuit should function once all components installed. VOX-Gain VR controls the sensitivity of the vox to the audio from the computer audio (line-out). Anti-vox gain is set at maximum, it can only be adjusted by altering volume of the receiver. It is also recommended (not seen in the circuit) to have a monitor speaker connected to the receiver's audio as the normal built-in speaker of the receive section will be disconnected once the radio is plugged.

VOX delay can be fine tuned by altering the RC value at pin 2 of the MC14538. Current value 1M verses 1uf contributes to a 1 sec.delay. The larger the R or C value, the longer the delay.

The reverse output of each mono-stable are connected to the unit enable input of its counter partner. For example, the vox gain path output is used to suppress the enable of the anti-vox seciton and vice versa. This is done by the 100K and the diode combination. The circuit provides a different between charge and discharge of the 1uf capacitor at the enable pin. This creates a fast disable but delayed enable action of either section. This is essential to prevent oscillation of the transmit circuit due to noisy trigger or DC pulsing pick up from the speaker wire.   

During initial set up, make sure the mixer panel of the computer is set up as follows :

Record panel : enable line in, microphone or other normal audio entry as required.

Monitor panel : enable any input except line out.

Change the 220K resistor between pin 5,6 of the LM4558 to a higher value if the anti-vox cannot suppress a transmission with normal listening receive volume at the monitor speaker.



The printed circuit board of this VOX is available from me. Email for more detail.


Patrick Wong Sep-97.


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