# How to capacity threshold dectector with a simple analogic circuit?

I try to make an extremely simple capacitive proximity sensor using a metallic plate and a plastic sheet, to show how it works. My aim is to put the capacitor in a circuit that lights a lamp (or a LED) if its capacity C is above some critical value C0 and turns it off when C is smaller than C0 (the order of magnitude of the capacities is 1-100 pF. Now the problem is to keep the circuit as simple as possible, so that it is easy to understand (and fits in what my target already knows). I especially want to avoid microcontrollers (thus an Arduino setup like in Triggering a capacitive sensor electronically? is not possible) - the most advanced device I can use is an op amp.

All I have now is a AC tension divider followed by a rectifier, but it has grown because I have to amplify the output then compare it with some reference voltage to turn on and off the lamp.

Do you have any idea ?

Thanks.

I would use the capacitor as frequency determining element of an oscillator (i.e. its frequency will depend on the measured proximity) and then determine whether the oscillator frequency is above or below the threshold.

The are probaly several good ways to do the latter, e.g.

• employing a simple f/V converter (e.g. one-shot and low pass filter) plus an comparator or
• (though not an analog solution) using a digital counter that gets periodically reset by a signal of fixed reference frequency and checking whether highest counter bit gets high eventually:

An omp-amp comparator compares the voltage to a fixed value. Simple and easy to use.

• Well, yes, it is what I use now, but I have to go through the AC tension divider and the rectifier before, and I wanted to now if there are other methods than that, hopefully more simple. Anyway, thanks. May 9 '12 at 20:49
• Use a bridge setup. Two branches that are in balance while in rest. One of the brances is made out of static components and the other branch is identical, apart from the capacity. The difference voltage from the bridge can easily be fed into the opamp. I think you need to configure it into a window comparator. May 9 '12 at 20:57

Leaving quite a bit of room for improvement, but very simply understood is the circuit below. Two voltage dividers powered by an AC voltage, one with a fixed capacitance, the other with a variable one. Diodes are added to avoid negative voltages. The voltage difference between both dividers can be used to drive a LED.

Compare the RC rise time of the capacitance to be sensed relative to a reference RC. Measure the difference with a D flip flop (DFF).

C2 is the capacitance to be sensed. C1 is the reference capacitance. Node 'tog' toggles between high and low. When 'tog' goes low, 'tri' and 'sns' race to get lower than 'hyst'. If 'sns' gets there first - because C2 is less than C1 - then the DFF U2A will clock in a zero. If 'sns' does not get there first, because C2 is greater than C1, then the DFF will clock in a 1. So pQ goes high when C2 is less than C1 - below 100 pF. nQ goes high when C2 is greater than C1, above 100 pF.

Notes:
- no micro as requested
- opamps can be substituted with low power R2R SOT23 opamps
- DFF can be substituted with SN74LVC2G74 SOIC8 DFF
- total power can be below 1 mA
- 3.3 VDC supply used as an example. Still works with 5 VDC supply.
- D1-R6 quickly charges C2 to get ready for the next cycle
- Does not use schmitt inputs to get hysteresis - so it's not dependent on input trigger voltage variation for accuracy. Both opamps trigger at the SAME voltage "hyst', which you could control by adjusting R1.
- normal supply bypassing and power supplies not modeled for clarity

Schmitt input trigger voltages can vary slightly between different inputs on the same chip, and can vary with temperature. This circuit offers an alternative to schmitt inputs for precision electronics.

Demonstration with C2 at 70 pF.

Demonstration with C2 at 120 pF.

Hope this helps.

If I could convince you to use another technology that only costs \$2.69 for (1) can run off a cheap 3V Lithium coin for a year has high noise immunity and with two units will extend to 20cm range?

The part number is stock at DK. Get the coin cell and holder too

I vote for the HSDL-9100 Integrated Reflective Proximity Sensor. trigger it with your finger or wave your hand swish.... Then use a D Flip Flop Q# to D to toggle inputs if you want it to latch the on off state.