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I have build a stepped tone generator (aka Atari Punk console) which are 2 555 timers connected together (one 555 is configured as a astable oscillator and drives the second configured as a monostable oscillator; more info here). APC diagram

The circuit works as expected (I am using a 556 in place of the two 555).

I would like to control the frequency of the 555s from a midi keyboard. For this I would like to use a MIDI to CV converter that translates midi signals to voltages. I also build this circuit, which is based on an arduino and 2 DACs (MCP4922) and works as expected. (I can control another analog synth with it).

However, if I connect the MIDI to CV converter to the CONTROL pin of the 555, it does not work: whatever the output of the DAC, I measure around 5V on the CTRL pin. I believe this comes from the fact that the CONTROL pin of the 555 is at a voltage of 2/3 Vcc by default. How can i control the voltage applied on the control pin of the 555s with such a system ?

I am relatively new to electronics and this might be a complete newbie question. I am sorry if this is the case but I could not find the answer anywhere on the internet.

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  • \$\begingroup\$ What's the supply voltage on your 556? If it's more than 7.5V, then the control pin will be higher than the 5V supply of your DAC, and the DAC's pin protection diodes are clamping the voltage. \$\endgroup\$
    – Dave Tweed
    Feb 1, 2015 at 16:43
  • \$\begingroup\$ Fine project, but keep in mind you don't need the 555s at all if you have a microcontroller in the loop. You can pretty much do the punk console in firmware. \$\endgroup\$
    – Scott Seidman
    Feb 1, 2015 at 18:20
  • \$\begingroup\$ @DaveTweed: The supply voltage of the 556 is ca. 9V and the voltage on the CTRL pin is around 5.8V. \$\endgroup\$
    – xraynaud
    Feb 1, 2015 at 19:04
  • \$\begingroup\$ @ScottSeidman: I know, but I'm trying to build a complete modular synth from scratch with full analog and digital parts. I use the MIDItoCV module to control the synth from a computer but I'm planning to have a full working analog modular synth one day \$\endgroup\$
    – xraynaud
    Feb 1, 2015 at 19:07

1 Answer 1

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The control pin of the 555 will need to be driven with a signal from a fairly low impedance source. The data sheet of the 555 will show you the range of control voltage that will be effective. If the control range needed does not match the voltage generated out of your MIDI-2-VOLTAGE converter then you may find it necessary to adjust the DAC output range accordingly. Do check the output impedance of your DACs and make sure that they are at least 10 times lower impedance then the control pin input pin of the 555's. If this is not the case you will have to use some op-amp voltage followers to buffer between the DAC and the 555.

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  • \$\begingroup\$ Thanks for your answer. I added 2 voltage follower opamps and it works now. \$\endgroup\$
    – xraynaud
    Feb 1, 2015 at 19:09
  • \$\begingroup\$ @xraynaud. I'm having the same issue with LFO control of a 555 oscillator. Did you put one voltage follower right after the other? Did you locate them before the output of you cv source (on the cv side) or after (on the 555 side)? \$\endgroup\$
    – kevtrout
    Jan 11, 2016 at 1:47

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