v097pba8@ubvmsd.cc.buffalo.edu (Ken F Morton) (02/07/91)
I think I figured out my little problem last night, and I wonder if anyone could tell me what you think of it. To recap, what I needed was to take the voltage output of a piezo element and 1) Close one contact 2) Use the voltage from the piezo to generate a delay time after which a second contact would be closed. What this would be use for is drum pad interface. Basically replacing the keys of a cheap velocity sensitive MIDI keyboard with hacked drum pads. That way I could slave a drum machine to the keyboard, and play the drums on the pad. Velocity sensitivity on boards works by measuring the delay between two switch toggles. This value is then converted to MIDI velocity data (0-127). Now here's the basic idea of the circuit: The piezo element drives a optocoupler. The optocoupler photoresistor end acts as the feed resistor in a 555 timer circuit. (The more voltage (harder hit) from the piezo, the less resistance in the photoresistor, therefore the capacitor in the 555 circuit charges faster producing a shorter delay.) The 555 circuit drives a fast relay, holding the relay open on one side during the delay time. When the relay contacts the normally open side (when the coil isn't charged) it triggers a 555 bounceless switch which closes the first contact for a small time. When the delay is over and the relay opens again it closes a second 555 bounceless switch which then triggers the second contact. That's the circuit. I basically got it all from Forrest M. Mims III "Engineers Notebook II" from Radio Shack (Archer). It's the old yellow one. Does anyone remember these things? I don't think they make them anymore. There are still some incertainties on my part about sensing of velocity on keyboards. I don't know if the first contact must remain closed before the second is hit or whatever. I'll just have to get a hold of one and hack on it. My only concern is the speed of the circuit. With three relays there might be unwanted delays. Tell me what you think. thanks... Ken Morton v097pba8@ubvmsd.cc.buffalo.edu