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

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

• Also any signal will have some noise so very low voltage triggering should be avoided. You will have to decide on a threshold voltage. Feb 10 '20 at 21:37
• Show us the schematic you tried. This website has a schematic editor or you can just draw a picture. You said it works with an Arduino right? So you can just inject 3V or 5V into the main alarm system and it will work? To detect 150mV you probably need a comparator. You don't really need your relays, optos, or FETs but you can have them if you want additional drive current or isolation. Can you provide more details about your main alarm input? Feb 10 '20 at 21:44
• what is a museum cabinet detector? ... how do you know that it is fully functional? Feb 10 '20 at 22:21
• @jsotola: It's a device that detects museum cabinets, of course. I don't need one. We've never had a problem with museum cabinets in my house. Feb 10 '20 at 22:48
• What type of signal needs to go into the alarm system? What voltage? And what current? Does it need enough current to drive a relay or something? Or does it have to be a relay that connects two contacts of the alarm system together (like you have in your drawing with the 4N35) Feb 11 '20 at 20:44

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.

• Thank you very very much, I will try to see how it works in real. But it makes sense, of course.. It's really different building my own schematic from just reading something. wow
– YJL
Feb 11 '20 at 21:19
• @YJL I got lazy and did not size R3 for you. it depends on what you want for R1 and R2 anyways. You can read here how to do it: rohmfs.rohm.com/en/products/databook/applinote/ic/amp_linear/… Feb 11 '20 at 21:21
• Thank you, I appreciate it.
– YJL
Feb 11 '20 at 21:22
• @YJL This one might make more sense. Most important is Figure 4: ti.com/lit/ug/tidu020a/tidu020a.pdf Feb 11 '20 at 21:24

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 .

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.