The attached circuit generates a AC coupled quad square into a high impedance load. It costs roughly 6$ to build including the battery. Output is 30 hz +/- 5 hz due to capacitor tolerances. Very useful as a sanity check in emergencies. I will build a more professional level shifted, DC coupled, ILDA compatable version if there is demand. I retain the copyright to this circuit. PhotonLexiconers may build one each for personal use.
If you use a 74LS76 or 7476 TTL you get a 1.8 volt peak to peak output. If you use a 74HC76 you get a 4.8 volt peak to peak output.
Drawn in wonderful BMP "Walleye Vision" (Fisheye Vision for non North Americans) so people who have few electronics skills can understand it.
You get 4 dots on the wall in a square, The faster your galvos, the less visible connection between the dots.
This circuit is useful for when you are using sound card dacs and you dont know if it is the dac or the galvos that are messed up.
There are two Y outputs, one rotates clockwise, one counterclockwise, not that you can tell at 30 hz. use a mylar or ceramic cap for c3, c4, c5, unless you are sure your capcitor is non-polar. A wide range of values will work. If you do not install the capcitors you get a nasty DC offset.
For high to medium impedance inputs, ie most galvo amps.
I'm NOT responsible if you blow up your galvos. If this circuit is correctly built and powered off a battery that is nearly impossible to do.
I realize this circuit is a joke to a engineering professionals, but in a pinch has worked well for emergency field troubleshooting. I literally built it in 20 minutes in a friends shop, when a scope was not available to test a pair of Cambridges.
For purists, this is a walking ring counter using a dual JK Flip-Flop with AC coupled outputs. F out is a quadrature square wave at 1/4 the input frequency.
Steve