Honeywell DT-7235T

Hi all. My parents recently got a new security system and gave me all the parts from the old one (mostly the RF and button panels, but also this motion sensor). I'm slowly figuring out how to use the different components and get them hooked up to microcontrollers for my own silly experiments.

I've been playing with this motion sensor over the weekend and I'm pretty perplexed (which is admittedly easy with this stuff). I've found marketing infos and datasheet as well as an installation guide but I don't know how to interpret them.

I can power it and it seems to be working fine, since it has an indicator LED when it picks up motion, but I don't know how to get the trigger signal from it. If you look at the installation guide, you'll see they describe the the connections as: - Alarm relay: Energized Form A 500 mA, 30VDC (the installation guide and stickers on the unit label the relay connections as C and NC) - Tamper: (NC) 50 mA, 24VDC (two connections, each labelled T)

I've googled all sorts of variations of "Alarm relay energized form A", but I haven't found anything that explains what that really means (in words I can understand).

The first thing I did was connect a multimeter to test the relay connections. I tried every combo of ground and power that I could think of, but when I connect ground to the power ground terminal and positive to either C or NC, I get varied voltages depending on the state. It seems that when it's not detecting, those terminals output .6V, but when it does sense motion, they drop to .3-.4V.

That seems fairly straight forward, but I don't really know what to do with that information. When I connect to an analog input, I get readings all over the place (from 0 to 0.99) but they don't sync with when the indicator light is showing it detected motion. So, I thought, maybe I misinterpreted and should use a digital input instead. Well, that's just as random, bouncing from 0 to 1, seemingly with no relation to when it's detecting motion.

I'm sure this is much simpler than I'm making it, but I'm lost. Can anyone give me any advice on how to use this sensor?


Hi Bluecamel!

I have the same sensor, and want exactly the same as you.

Did you finde the solution?? Becouse i'm getting crazy!

Ok there is the power supply ,S spotted 12v dc . There are two ( maybe 3) really contacts which operate when something is detected - usually labelled ‘c ‘ common ,’ N/O’ normally open contact and/or ‘n/c’ normally closed - just connect between ‘c’ and one if the others as though this is a switch in your circuit. The tamper loop you can ignore - these sensors are wired with a six core cable , if someone cuts the cable the cable haa been tampered with ! And the tamper loop broken .

Many thanks!

I have connected the power supply positive and negative and the sensor works fine. The thing is connect it to the arduino.

I have try to connect the NC directly to arduino and read the input pin, to see what happens when the sensor it's activated, but nothing happens.

Then a try to connect NC to C with a resistance and then to the arduino, but nothing..

What i'm doing wrong? I'll try to post pictures of the systems..

Many thanks!!

Ok I Got it!

That's the solution,

  • -> 12V

  • -> GND

C -> 12V

NC -> Pin 2

and then the code for arduino it's

const int pir = 2;

const int ledPin =  13;

int pirState = 0;

void setup() {

  pinMode(ledPin, OUTPUT);

  pinMode(pir, INPUT);

void loop() {

  pirState = digitalRead(pir);

  if (pirState == HIGH) {
    // turn LED on:
    digitalWrite(ledPin, HIGH);
  else {
    // turn LED off:
    digitalWrite(ledPin, LOW);

Hi Miguelillo,
I don’t have the same PIR sensor as you (mine is an ABUS BW8000) but I’m having similar problems to the original poster , where the LED on the sensor indicates that its detecting movement but I can’t figure out how to connect it up to the arduino.

I have two connections which are just labeled alarm and I’m hoping that they do the same as C and NC in your case. When you say C → 12V, how do you have this wired? Sorry if the answer is obvious, I’m new to this and don’t want to fry my arduino!