This circuit is a voice operated switch (VOX) and designed for audio frequencies within a commercial telephone bandwidth of
100Hz to 3000Hz. Input amplitude is sensitive down to 10mV and the output comparator can drive a relay or interface to micro
processors and TTL switching between 0 and 5Vdc.
The VOX circuit is shown above and drawn in LTspice. The input is from a dynamic microphone and typical speech amplitudes are in the
mV range. The input is sensitive down to 10mV peak to peak. In the simulation circuit, the microphone is replaced by voltage
source V1 (shown only in the simulation circuit).
The output waveforms for a typical audio signal are shown below. A sample of speech or packet lasts for 3.2 seconds.
The circuit will trigger high after 40ms and switch low 150ms after the input signal ends.
The audio input is full wave rectified by C1, D1 and D2. The output is further filtered by R7 and C5 before being applied to
the comparator. R10 and C3 are the load and under no signal (no audio input) C3 is discharged via R11.
The LM7321/LM7321Q provides further output buffering and can drive loads up to 30mA.
VOX Circuit with Microprocessor Interface
Another application for this circuit is to monitor water flowing through a pipe. The microphone would be placed close to the
pipe and some foam wrapped around microphone and pipe to filter out background noise. Any water flowing in the pipe would trigger
the comparator. The comparator can be reset by applying a positive 5V voltage to R8, which will rapidly discharge C3 via Q2.
Q2 can be any general purpose transistor such as 2N2222, BC108 etc. The modified circuit is shown below:
V2 is the power supply source and 5V is chosen to interface directly with PIC microcontrollers and TTL IC’s etc..
V3 is a volatge source representing the output signal from a microprocessor. The microprocessor applies an output signal for
10ms which will discharge C3. If water is still flowing in the pipe, and heard by the microphone, then C3 will start to charge
again and output of comparator will go high. This way the microprocessor knows there is still water flowing in the pipe.
The vox output (vox_o/p) in the waveform diagram, is fed to a fast comparator, the LTC6702. The comparator output is jitter free
and swings between 0 and 5V. This can directly drive a PIC, TTL or microprocessor. If more output current is desired then buffering
with an op-amp configured as a voltage follower that can sink and source 100mA will drive just about any output load.
Valid input signals from the microphone are in the range 10 to 100mV
1. Starting from the left, there is no voice audio input, no squiggly trace along 0V line.
2. The microphone signal is represented by the light blue 20mV trace. The input is continuous to the right
hand end of the graph. As microphone is listening to a pipe, this indicates water flow for the rest of the time on the x-axis.
3. V(vox_o/p) …rises with an input signal (2, above). The V(vox_o/p triggers at about 0.6V and in turn causes the comparator
to change state. The output called (hi_flow, red trace) rises from 0V to + 5V.
4. Looking at 0.2sec on the x-axis, the v(discharge ) curve is closely following the vox o/p voltage. It is an open collector.
5 However when the microprocessor wants to know if the audio is still present (water flow in the pipe) it sends a 10ms
pulse which we see as a very sharp fall to zero, discharging C 3.3uF.
6. This discharge resets Vhi_flow to zero. However, if water is still flowing, noise will be picked up by the microphone
7. The Vox bluen line can start to rise again and where it goes above the comparator trigger level, Vhi_flow toggles to +5V.
8. This brief discharge sending 0 from the microprocessor and 40ms later returns to 5V ..informs the microprocessor that audio
is still present.
9. We see on the graph that the sampling discharge happens about once a second.
10. For safety we could sample every 100ms