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Voice activated audio switch for FM wireless microphones This VOX circuit is intended for use with FM wireless microphone circuits & will provide audio muting of the transmit­ter section. It uses just one CMOS IC & a handful of transistors. By DARREN YATES There are two common applications for a voice-operated switch or VOX as it is commonly called. The first is to stop and start a tape recorder so that it runs only when a voice or sounds are present to be recorded. Second, a VOX is commonly used to take the place of the press-to-talk switch on transmitters as used for amateur radio communications or in hands-free cellular phones in cars. 56  Silicon Chip The VOX presented here differs slightly in that it doesn’t switch a relay but it switches the audio output on and off. When you speak, a CMOS switch closes and stays closed until about 1.5 seconds after you stop speaking. As such it could be teamed with the FM Wireless Microphone project described in the October 1993 issue of SILICON CHIP. This would have the advantage of more professional opera- tion as the wireless microphone would not pick up noise when you were not speaking. In effect, the VOX circuit provides audio muting for the transmitter. This should not be confused with the muting feature commonly incorporated into FM tuners so let’s explain the princi­ple a little further. Most FM tuners such as those used in hifi systems have muting. This has two effects. It prevents the tuner from produc­ing copious hiss when being tuned between stations and it also mutes the audio when the received signal strength drops below a set level which is usually around 10 microvolts or thereabouts. On the other hand, when you are using an FM wireless micro­ phone, you usually would disable the muting feature on the tuner. If not, you could have the annoyance of the audio being muted on and off as the speaker moves D3 1N4004 2.2k 10k 10 10 10k Q2 BC558 B 100k 0.1 Q1 BC548 B 100k C C 220k E 180k MIC 4.7k E 0.1 10k 470pF Q3 BC548 B 0.1 Q4 BC548 2x1N914 D2 1M B C D1 2.2 1M 390  330  IC1b 14 C 4 5 3 IC1a 4066 E E 100k 6-15V REG. 100 16VW 100k 6.8k 13 2 1 47k 7 47k 2.2 100k OUTPUT 2.2k B E C VIEWED FROM BELOW 10 16VW 10 VOICE ACTIVATED AUDIO SWITCH Fig.1: the electret microphone picks up the audio signal & feeds it to a preamplifier stage consisting of Q1 & Q2. From here, the signal is fed via two paths: (1) to Q3 & (2) to CMOS switch IC1a. Q3 drives a voltage doubler circuit based on D1 & D2. When a signal is present, Q4 turns on & drives CMOS switch IC1b which in turn closes IC1a to switch the audio signal through to the output. around and causes the FM signal to fluctuate. So with the receiver (read: tuner) wide open all the time, it will reproduce all noises picked up by the microphone whether or not the speaker is talking. Now it’s no good having a VOX circuit to turn the FM wireless microphone transmitter on and off. If the transmitter is turned off, the tuner will immediately produce hiss; lots of it. Hence the transmitter must run continu­ously to keep the tuner quieted (ie, not producing hiss) but the audio preamplifier must be muted. That is the purpose of the VOX circuit presented here. Circuit diagram Looking at the circuit diagram in Fig.1, the electret microphone insert picks up the audio signal which is then fed to the preamplifier consisting of transistors Q1, Q2 and their associated components. The gain of this preamplifier is set to 33 by the 10kΩ negative feedback resistor and the 330Ω resistor con­nected to the emitter to Q1. To make sure that the amplifier doesn’t amplify RF signals, a 470pF capacitor across the 10kΩ feedback resistor limits the upper frequency response (-3dB down) to 33kHz. The output of the preamplifier is taken from the 2.2kΩ collector resistor of Q2. From here, the signal takes two paths. First, it is amplified by Q3 which has a gain of about 12. Its output drives a diode voltage doubler using diodes D1 and D2, as well as the 0.1µF and 2.2µF capacitors. PARTS LIST 1 PC board, code 01203941, 118 x 51mm 1 electret mic insert 4 PC pins Semiconductors 1 4066 CMOS quad analog switch (IC1) 3 BC548 NPN transistors (Q1,Q3,Q4) 1 BC558 PNP transistor (Q2) 2 1N914 signal diodes (D1,D2) 1 1N4004 rectifier diode (D3) Capacitors 1 100µF 16VW electrolytic 4 10µF 16VW electrolytic 2 2.2µF 63VW electrolytic 3 0.1µF MKT polyester 1 470pF 63VW MKT polyester Resistors (0.25W, 1%) 2 1MΩ 3 10kΩ 1 220kΩ 1 6.8kΩ 1 180kΩ 1 4.7kΩ 5 100kΩ 2 2.2kΩ 2 47kΩ 1 390Ω Miscellaneous Tinned copper wire (for link), plastic case, solder What we end up with across the 2.2µF capacitor is a DC voltage of around 8-9V whenever a signal of sufficient loud­ness is picked up by the microphone. This voltage is used to turn on transistor Q4 which in turn drives IC1b which is one-quarter of a 4066 CMOS analog switch. Finally, the voltage from pin 4 of IC1b is used as the control signal for IC1a and this switches the audio signal from the collector of Q2 through the to the output. To make sure that no clicks or plops occur when switching, two 47kΩ resistors and a 10µF capacitor equalise the DC on both sides of the switch. The 10µF capacitor shunts AC signals to ground which would otherwise be fed through the 47kΩ resistors to the output. Power supply Just about any power source from 6-15VDC can be used. If you intend using the circuit in conjunction with the FM Wireless Microphone you can use the same 9V battery supply. Diode D1 provides reverse polarity protection while the 100µF capacitor provides supply bypassing. Construction All of the components for the Voice-operated Audio Switch are installed on a PC board measuring 118 x 51mm and coded 01203941. Before you begin any soldering, check the board carefully for any shorts or breaks in the copper tracks by comparing it with the published artwork. Once you’re satisfied that everything looks correct, start by March 1994  57 220k 2.2k 10k 10uF 1M D2 1M 390  100k 10uF IC1 4066 Q4 D3 330  180k Q1 0.1 47k 47k 100k 4.7k 100k Q3 1 470pF MIC 0.1 Q2 0.1 10k 6.8k 100k 10k 2.2k 10uF D1 2.2uF 10uF 100k O/P 100uF GND 6-15V 2.2uF installing the single wire link and then continue with the resistors. If you are unable to distinguish the colour bands on the resistors (which is quite possible with some brands of 1% resistors), use a multimeter to check the resistance values. Now solder in the three diodes, followed by the transistors and the IC. Take care with the transistors since Q2 is a PNP type, while Q1, Q3 & Q4 are all NPN types. Make sure that you install them correctly, otherwise the circuit will not work, or worse, the transistor may be damaged. Lastly, solder in the capacitors, Fig.2: make sure that all polarised parts are correctly oriented during the PC board assembly. Note also that Q2 is a PNP type while the remaining transistors are all NPN types. the microphone insert and the four PC stakes. When you have finished installing the compon­ents, check for any solder splashes on the underside of the PC board which could cause shorts between the tracks. If you find any, clean them off with your soldering iron. Testing Now for the smoke test. Connect up your power supply in series with your multimeter on a low milliamps range – around 100-200mA is ideal. When you switch the power on, you should get a current consumption of around Fig.3: this is the full-size etching pattern for the PC board 10mA. Any more than this, and you should switch off and check the board carefully against the overlay wiring diagram. You may have a component installed in the wrong place or in the wrong way around. If it passes the smoke test, take your multimeter and measure the voltage at pin 13 of IC1a. You may find it easier to go back to the 100kΩ resistor connected to pin 4 of IC1b. If when you speak at normal volume, the voltage quickly rises up to somewhere near the supply voltage, then all is OK. When all is quiet (and it may need to be fairly quiet), the voltage should drop to 0V after about two or three seconds. Lastly, to check that the audio signal is being switched, connect the audio output to a signal amplifier (the CHAMP low-power amplifier published back in the February 1994 issue is ideal), then speak and listen for the audio to switch in and out. If this appears to be OK, then you should be right. Connecting up If you are building this project for use with the FM Wireless Microphone published in the October 1993 issue of SILI­CON CHIP, you will need to make several minor modifications. These involve omitting the electret, transistor Q1 and their associated components from the wireless microphone circuit and then coupling the output signal from the VOX circuit into the 8.2kΩ input resistor for Q3. If you need to adjust the audio gain, this can be reduced by decreasing the 10kΩ feedback resistor from Q2 on the SC VOX board. RESISTOR COLOUR CODES ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ ❏ No. 2 1 1 5 2 3 1 1 2 1 58  Silicon Chip Value 1MΩ 220kΩ 180kΩ 100kΩ 47kΩ 10kΩ 6.8kΩ 4.7kΩ 2.2kΩ 390Ω 4-Band Code (1%) brown black green brown red red yellow brown brown grey yellow brown brown black yellow brown yellow violet orange brown brown black orange brown blue grey red brown yellow violet red brown red red red brown orange white brown brown 5-Band Code (1%) brown black black yellow brown red red black orange brown brown grey black orange brown brown black black orange brown yellow violet black red brown brown black black red brown blue grey black brown brown yellow violet black brown brown red red black brown brown orange white black black brown