Non intrusive pickup of PWM signal? - Page 1
Forum section: electronics / beginners

author: Rack201
author_rank: Contributor
timestamp: April 28, 2025, 08:40:16 pm
content: Hi - I’d like to add a sensor to my diy OBD2 monitoring project in my car that shows the state of a specific transmission solenoid. The signal from the cars ECU to this solenoid is PWM, and I don’t want to cut into the ECU loom. Wondering if it’s possible to sense the PWM signal non intrusively? Would a current sense transformer or Hall effect sensor pickup anything from a PWM signal? Or any other ideas?The peaks on the PWM signal are around 12v. While there is some modulation of the solenoid between off and on, I’m really just interested in detecting its fully on vs fully off.Cheers

author: langwadt
author_rank: Super Contributor
timestamp: April 28, 2025, 08:56:40 pm
content: if you can get connectors, make a short extension cable

author: incf
author_rank: Frequent Contributor
timestamp: April 28, 2025, 09:01:47 pm
content: An insulation displacement splice connector - if you have access to the individual wire in question.

author: David Hess
author_rank: Super Contributor
timestamp: April 29, 2025, 02:18:42 am
content: A small current transformer will be able to detect the signal since considerably current is involved in driving a solenoid. This would mean pulling the wire out of the loom but not cutting it. The current transformer would need to be the clamp kind so that the wire does not need to be cut.

author: Poroit
author_rank: Frequent Contributor
timestamp: April 29, 2025, 08:59:11 am
content: G'day Rack201,Have a look at Fastron's range.https://fastron.com.au/collections/hall-effect-current-sensors/products/hct-0010-010

author: PtrH
author_rank: Newbie
timestamp: June 16, 2025, 05:19:10 pm
content: A small Hall sensor like ours (https://asensor.eu/products/HE144) might work. But your fields might be too smal. Do you know your magnetic field?Otherwise a pickup coil should work, I am sure the protocol works without DC level (meaning NRZI). Otherwise that system would not work.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 17, 2025, 08:44:58 am
content: It's worth pointing out that you would be monitoring thedriveto the solenoid, not the solenoid. I mention it because those solenoids can get sluggish, stick, or fail completely, and you wouldn't pick that up from the drive signal.

author: Analog Kid
author_rank: Super Contributor
timestamp: June 17, 2025, 09:06:40 am
content: Quote from: SteveThackery on June 17, 2025, 08:44:58 amYabbut, the only way to monitor the actual solenoid operation would be mechanically, which I'm sure the OP is aware of anyhow.

author: SteveThackery
author_rank: Super Contributor
timestamp: June 17, 2025, 05:42:20 pm
content: Quote from: Analog Kid on June 17, 2025, 09:06:40 amMaybe! I did wonder if the OP was chasing down a fault arising from the non-operation of the solenoid, and hadn't thought through the mechanical failure modes.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 17, 2025, 06:08:01 pm
content: I once made a capacitively coupled zero crossing on mains wiring. For the "capacitors" I used 10cm long pieces of (also) insulated wire that was taped to the corresponding wire of the mains wiring, I did not damage the insulation of the mains wiring in any way. From these two capacitors, I made a differential amplifier with a generic opamp. Make sure that the inputs of your opamp have a DC path to GND, or they won't work, even if bias current is in the pA range, it's still a real current.