Teardown: Bosch DS835 TriTech PIR/Microwave Motion Detector

One of the things I bought home from the recent Wyong Field Day was a Bosch DS835 TriTech motion detector. While the box was beat-up, the unit inside looked clean and the seller wanted just $5 for it, so I decided to give it a new home.

The Unit

The box itself is quite scuffed, but there’s no mistake – this is a Bosch Security Systems DS835 TriTech PIR/Microwave Motion Detector. From the date code of 886, it appears to be made in June 2008 in China.

Inside, there is a copy of the installation instructions and the detector itself. The front face has the PIR window, an LED window and a blank spot where a logo badge would normally sit. Part of the reason I purchased this detector aside from the TriTech name was because a very similar model is installed in our own house.

This is where the DS835 model name comes in – that area for the badge is actually occupied by a logo for Detection Systems on our older sensors. The Detection Systems company was purchased by Bosch in 2000, but the sensor still retains its “DS”-model designator as it is reputable.

A label at the top illustrates FCC compliance …

… the underside has a little gap for a look-down zone, and a clip latch release hole to open the detector.

The rear of the detector unit has not been breached, but has a number of areas for cables to enter/exit the unit. The unit also has bevelled edges for mounting in the corner of two walls. From this, I can only assume the detector was never installed.


To open the unit, it’s a simple matter of using a flat bladed screwdriver to release the latch and flip the unit open. The front cover shows a light-pipe for the LED indication and the PIR lens with many different segments. The lens itself can be masked from the inside, but isn’t adjustable otherwise.

Looking at the other side, it’s clear why – the top left corner has a black screw with a slot – to adjust the look-down angle, the whole PCB is slid up or down in the casing. Judging by the foam that is still inside, normally used to plug up the cable entry hole, it’s probably a brand-new sensor. We can see a limited amount of jumper-based configurability – sensitivity normal or intermediate and LED on/off. Microwave range is adjusted by a potentiometer.

There is the pyroelectric sensor which is responsible for the PIR segment, a microswitch that senses tampering with the case and the antenna for the microwave (~10GHz) radar. I suppose that could be where “TriTech” comes from – or maybe from the intelligent filtering algorithm that reduces false detections. A bi-colour LED provides feedback about which mode detected motion and whether it was tripped, with a self-test supervisory feature that reports with a blinking LED in case of microwave malfunction. Output is via a relay that is normally closed. The angled terminal blocks include some spare connections as well – but the tamper circuit and relay outputs are separate, with 12V (nominal) input on the left-most side.

Removing the whole PCB shows the rear is covered by a black plastic cover. This makes sense as vermin could cause inadvertent short circuiting between points on the PCB. There is a loop at the top, which I suspect is to hold the cable as it loops over to the terminal block.

It is possible to remove the remainder of the screws to remove the black plastic cover, but I wouldn’t suggest doing that …

It can be seen that the microwave radar section actually has a lot of similarities to a Ku satellite LNB – this is a classic dielectric resonator oscillator as evidenced by the brown “slug” of ceramic next to an microwave transistor.

Correspondingly, on the rear cover, we can see the screw that impinges onto this area to tune the frequency, as in an LNB to tune the local oscillator. Another interesting finding was the unit being powered by a PIC16C711 8-bit microcontroller, which has just 1K EPROM and 68 bytes of RAM, but other than that, there’s not much else to see aside from the LM324 quad op-amp.


While I knew what to expect from the insides of a regular PIR sensor, opening a multi-technology microwave radar-based sensor was interesting. I didn’t expect the paths of satellite LNBs to cross with motion detectors – it seems they use fundamentally similar dielectric resonator oscillator technology to create their ~10GHz oscillations which depends on high speed transistors and very precisely shaped PCB traces. The screw in the rear cover would be used to tune the oscillation frequency so that it would be at a regulatory-legal frequency.

It was also surprising to note that the PIR with all its touted intelligent filtering of false triggering runs on a Microchip PIC16C711 8-bit microcontroller, with the whole sensor consuming just tens of milliamps.

About lui_gough

I'm a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!
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