Variable capacitance proximity sensor

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TomWaterman

Well-known member
Joined
Jun 4, 2004
Messages
1,151
Location
The Shire, UK
Hey, am posting for a colleague who has tried to reverse engineer a proximity sensor for use with some MIDI / MaxMSP live performance electronics.

This is the schemo he sent me after drawing it out from the PCB he's copying.

ProximitySensor.JPG


I can't make head nor tail of it and was wondering if it looks right? There are two plates (A&B) that form a capacitor and C varies as you move you hand closer like that of a theramin....but I'm (both of us) not clear on the transistor configs.

We would both be really grateful if one of you esteemed brains could shed some light on the circuit.

Cheers Tom
 
> not clear on the transistor configs.

Can NOT be right as drawn.

And I have real doubt about 1K across the sensor plates.

How can you measure capacitance? Either a large DC voltage (or a hyper-small DC output); or with an AC oscillator and some way to measure frequency or coupling.

Q1 Q2 are almost certainly wired as a multivibrator, probably collector to opposite base, with emitters running off to opposite supply rails (maybe through a resistor).

However, if they are honestly same type, that can't be right.

C1 D1 Q3 C3 is a tachometer (swap B and E). C1 passes a fixed charge on each oscillator cycle, pumping a fixed charge out of C3, sucking current from R2. For low oscillator frequency, C3 rises to 4.99V; for high frequency C3 falls to 0.1V. Frequency has something to do with capacitance around the plates.
 
Hi PRR - great to see you back around these parts, thanks for the reply!

I forwarded your reply to my colleague and I think he got the pin-out mixed up so all all bases need to be switched with the emitter in the above drawing.

I have a new drawing I can post when I get a chance to make a pic of it.
The transistors are definitely all one type - BC184LCs.

And there is 1k across the plates for sure - I had a look at the original board he's copying and that apparently works.

Forgive me - I'll look into it more when I've caught up on some much needed sleep - he's building one up over the weekend with the changes.

Thanks again
Tom
 

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