Most simple way to detect small voltage changes(~150mV)

Question

I have one unit of museum cabinet detector that works on 18v. I want make simple security circuit with detector. I've figured out that two output ports on detector are normally contact(NC), but it'll shows ~150mV difference charge when it sensed something. So main alarm system get signal. I have only units and I need to control it with my circuit with battery.

I assume It is quite simple logic so I sorted out with arduino at the moment. (Analog input from the detector, digital output to latch up alarm circuit. Alarm rings whenever it get signals for 3 seconds.)

But I believe there must be much simpler way to build it by only electronic components. I tried to sense voltage changing of the detector with FET, Opto-coupler, relay but has failed. Probably my component's specification doesn't match well with the situation but also I am not sure if I approach proper way.

When activated voltage changes 0~150mv what would be best and cost efficient way to detect the signal so that alarm circuit could get it as input?

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

You can do something like this:

^{simulate this circuit – Schematic created using CircuitLab}

I have included both a relay output and an opto output, depending on what you want to use.

• R3 is for hysteresis for clean switching.
• Because R3 must connect from output to non-inverting input for hysteresis, the reference divider must go on the non-inverting input which makes the output signal reversed from what is required to switch M1 and M2. M3 and R6 exist to reverse the logic output.
• D1 is a flyback diode because when you interrupt current through an inductor, it makes voltage spikes that destroys things if you don't give the current in the inductor a path to decay.
• R1 lets you tune the switching point. R2 is just so you have more resolution in your tuning since you are only interested in adjusting around 150mV.
• R4 is only required if the relay coil voltage is lower than your supply voltage. If they match you don't need it.
• You will still need a regulator of some kind since the accuracy of the switching threshold of R1 is dependent on V1. Do not power your relay off the regulator if you use one. You may or may not power the opto off the regulator if you want.

This circuit won't wait 3 seconds.

Answer 2

You are wanting to detect Changes in Voltage ,not absolute voltage .This means that you can capacitively couple to remove the nominal 18VDC component .You can easily amplify the AC signal that represents changes in voltage .A simple transistor or opamp will do to give a gain of say 10 .This nominal 1.5 Volt peak signal could go into an analog input for processing .

Answer 3

If the signal is relatively clean, 150mV might be fine to work with. Sample it with Analog.read on A0, spit the number to the serial port, and view it in the Serial monitor in the Arduino IDE. This might be exactly what the Analog.read example in the Arduino environment does.

In the Serial monitor, sample with the detector signal off for a while. You will see integers nearish to zero on your screen. Then set the detector output to it's high state and watch the numbers. They should be around 31 or so. If there is no overlap between the two steps, you can use an if statement to set D1, using a threshold of maybe 20.