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This time, the goal was to build a little instrument, to be played by hands, this kind of device is often called an opto-theremin…


In this case I build the circuit from scratch using a similar schematic as in my little hex-schmitt trigger synth, but instead of the IC 74c14 I used the 4093, which is a Quad 2-input NAND Schmitt trigger; This is a great IC to build oscillator-synths because it allows you to gate each output with a switch or with another oscillator…

here is a diagram of the 4093 IC connection pins:

4093 pin conections
4093 pin connections

and here is the schematic for the 4LDR synth:


note that i wrote the values of components guessing what i used because i don’t remember exactly, anyway you can play with different values to achive the sound performance that you prefer…. as it can be seen in the diagram, the Light dependent resistors are placed in series with little faders which allows to adjust the range of response of the LDRs;

what does what-ldr-synth

and here a video of me testing the little instrument, where you can hear all the r2d2-like sounds which this instrument is able to make…this little synth is one of the most fun and intuitive instruments i had build until now, and it is also my wife’s favourite…

by 9volts



  1. No specs on the diodes…are 9194s OK for this? Or do I need something more exotic?

    • i used 1n4148 ..but i’m pretty sure it will work

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