Here I describe how to build a simple low power tone generator that I use to teach my son morse code.
While cleaning up my cellar I found my old Wehrmacht morse keyer. This keyer was used by German forces during the second world war. The keyer had a strap attached with witch it could be fastened at the upper leg of the soldier. This way the soldier could use the keyer also when there was no table around. These keyers where also used by spies in combination with a small transmitter. Both parts where small enough that someone could carry them around without other noticing them.
My son found this old device very impressive and wanted to use it. So I tried to make a beeper that should have the following features;
- simple usage
- no power button
- very low power consumption
- speaker included, no headphones needed
- usage with 3V button cells
I tried several designs and came to the one that Dan posted for his "Light sensitive theremin" http://www.geocities.com/SoHo/Lofts/8713/optotheremin.html
. The device has nearly nothing to do with a real Theremin ( http://en.wikipedia.org/wiki/Theremin ) but it seemed to be exactly what I was looking for.
Step 1: BOM and Schematic
What you need:
- 1 * AC187K (or similar NPN transistor like the 2N3904)
- 1 * AC128K (or similar PNP transistor like the 2N3906)
- 1 * 50 kOhm trimmer resitor
- 1 * 4,7 kOhm resistor
- 1 * 0,1 uF capacitor (0,1uF is equal to 100nF)
- 1 * 1 uF electrolyth capacitor
- 1 * 8 Ohm computer speaker
- 1 * 3V button cell battery holder
It could be a little problematic to get the germanium transistors I used. I used them because I also found them in the cellar and liked to do something with them. Besides that, using the germanium transistors, the tone oscillator workes as low as 0,6V. Using silicium transistors, the voltage need to be a little bit higher than that. But in any case it should work with a 1,5V AAA battery.
The rest of the components is all standard. I sourced the PC speaker from an old desktop computer and also got the battery holder from the old mainboard. If you got an old computer that you want to throw away, just have a look inside. Maybe you can find all the components on the board somewhere.
Step 2: Building
After making some tests on a bread board I started soldering the circuits on a ex-board.
There are many soldering instructables here just search frofor "how to solder" to find some https://www.instructables.com/id/How-to-Solder-Videos%3a-Why-is-soldering-difficult-s/ .
There are not much components on the board. It should be easy to solder them. looking on the back side you can see that I needed one "bridge" but the rest is easy "routing". When using ex-boards I like to use the ones without stripes. I think that stripes are barriers that are like fences that block my "free routing ideas" :-)
I use the contact pins of the components to make the connections. Normally I do not need additional wires to do the connections. There are always "junk" pieces of wire than can be used to do the routing.
I avoided the soldering of the old germanium transistors right at the pins. I tried not to "overheat" them. That's why the transistor pins are bent through the holes but not soldered right there.
The keyer can be connected using the 2-pin header that I also got from an old PC-mainboard. The loud speaker is connected through two small header pins ... guess where it came from. :-)
Also the button cell battery comes from the ... you know ... PC-mainboard. After all these years there is still enough power in it to make the speaker beep.
Step 3: Result
The circuit draws about 60uA while using a 3V battery and not pressing the morse key. While pressing the key, the current consumption is about 10 mA.
The tone oscillator works starting at about 0,6V and I tested it until about 5V with no problems.
The frequency depends on the voltage. It might be a good Idea to use a potentiometer instead of a trimmer. The 1uF capacitor is used to get a "smooth keying". You can use other values if you like to vary the "smoothness".
Listening to the video you may think that the sound is a bit "rough". It's not that "rough", I used a small digicam for the recording of the video and the sound quality of that cam is not really a good one.