2019-01-08 99 Cent Wireless Microphone Kit Assembly

2018-01-08 also FB group Building Transistor Radios

Before the Xmas holidays I ordered a few of these at 99 cents apiece, and they gave a delivery date of late Jan to early Feb, so I was pleasantly surprised to get them early.

There are many of these single transistor microphones around, either as schematics or as kits. They have low audio sensitivity because the microphone puts out only millivolts and that’s not enough to fully modulate the RF oscillator. Blowing in the microphone is easily heard, but for your voice to be heard you have to talk loudly very close to the microphone.

This kit comes with a coin cell holder that will take a CR2032 3 volt lithium cell – it’s very compact, but the lifetime is not very long. I didn’t have a coin cell so I soldered a two AAA cell holder to the kit to try it out. All of the resistors are 1 percent, which is better, but the color bands are harder to read than 5% resistors. All of the capacitors are very low cost ceramic disks, and do not give the circuit good frequency stability.

This kit came with the hole in the PC board for a short antenna, but there was no wire for it, they assume you have a short piece of wire available – short meaning a foot or 300 mm. But when the antenna is used the frequency will change when the antenna is brought near other objects. Without the antenna the signal can be picked up a few tens of meters.

The coil comes already made, and measured 100 nanohenrys, which is typical for the low end of the FM band. The turns of the coil can be spread to increase the frequency, because the 30 pF tuning capacitor is fixed and can’t change the frequency. But this 30 pF value is on the very high end of the typical values found for the low end of the FM broadcast band. Also C5, the capacitor connected across the emitter and collector, is 10 pF, which is double the values typically used. In addition there is a capacitor C6 across the emitter resistor is usually not used, but in this case is 30 pF. All this adds up to more capacitance for this circuit and a lower frequency.

So after assembling the circuit I powered it up and measured the frequency with my Gooit frequency counter, and I got 78 MHz, more than 10 MHz too low. So I removed the 10 pF C5 and put a 5.6 pF in its place. I checked the frequency and it was still below 80 MHz. So I removed the 30 pF C6 and put the 10 pF in its place. The frequency then measured 81 MHz, still way too low. So I had to lower the 30 pF tuning capacitor C4 and put a 20 pF capacitor in its place. Now I measured 91 MHz, just about right. I can put a 0.5 to 5 pF variable capacitor across this 20 pF and move the frequency down to the 88 to 90 MHz part of the band.

The circuit needs another transistor to amplify the microphone so the listener can hear what is being said in the room. I will have to see if I can add a transistor and a few parts to make it more sensitive. As it is now it’s more of a kid’s toy, not really useful for listening.

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