Interfacing IR transistor with "AC tachometer"? - Page 1
Forum section: electronics / projects

author: daxliniere
author_rank: Regular Contributor
timestamp: March 28, 2025, 12:23:34 am
content: Hello all,I have a speed sensor module, commonly sold under the name "AC tachometer" which gets its name from the fact it runs from mains.It has 4-digit 7-segment LED display and comes with a hall-effect sensor for magnetic detection.I want to use it to measure the rotational speed of the shaft of a milling machine. The plan is to wrap the output shaft with matte black non-reflective tape for all but a tiny slit which would expose a shiny steel reflective patch.The NJK-5002C sensor that came with it is labeled as NPN, but in the limited tests I've done using an old phototransistor gate I had lying around, I couldn't get it to work.The sensor pins on the tach are labelled SIN, P+ and P-, though direct measurement with a multimeter doesn't seem to make much sense to me. (Perhaps there's already a pull-up resistor in there confusing the issue?)When I disassembled the included inductive sensor module, I noticed there are a few components, not simply the 3-pin hall-effect sensor.Can anyone suggest how this should be connected, please?All the best,Dax.

author: incf
author_rank: Frequent Contributor
timestamp: March 28, 2025, 12:40:11 am
content: Can you identify if any of the pins connect to the ground used by the linear regulator on the PCB? (same question for tracing out VCC)

author: floobydust
author_rank: Super Contributor
timestamp: March 28, 2025, 03:01:11 am
content: Quote from: daxliniere on March 28, 2025, 12:23:34 amWhat is this phototransistor, can you give some info.I did find:NJK-5002C Hall Effect Switch (has internal 10k pullup)BRN: +6-30V POWER (some spec 5-30V) to P+BLK: NPN OUTPUT to SINBLU: GND to P-ZSB 4-digit LED Tach ModuleSIN (Signal Input}P+ (+POWER) {unknown DC maybe 12V?)P- (GND)

author: daxliniere
author_rank: Regular Contributor
timestamp: March 28, 2025, 03:18:39 am
content: Quote from: floobydust on March 28, 2025, 03:01:11 amWas thinking this one because its beam is convergent which should make it better for targeting a small spot.https://www.farnell.com/datasheets/2331996.pdf

author: floobydust
author_rank: Super Contributor
timestamp: March 28, 2025, 03:38:32 am
content: That is a phototransistor plus LED.You have add a resistor to power the LED, and probably a pullup resistor for the phototransistor collector.We just don't know what DC power this tach module puts out. You'd have to measure DCV between P+ and P- with a multimeter.Pin 1 ORG LED(Anode +) to a resistor maybe 1kohm then to P+Pin 2 GRN LED(Cathode -) to P-Pin 4 WHT Phototransistor Collector to SIN and also a resistor, maybe 10kohm to P+ (or maybe the resistor is not needed, built into module)Pin 3 BLU Phototransistor Emitter P-An enemy to these photo sensors is ambient lights - sunlight, bay lights etc. can give interference.Best to have the right distance to the target 4mm and a light shield.

author: daxliniere
author_rank: Regular Contributor
timestamp: March 29, 2025, 01:06:13 am
content: Thanks Floobydust.I has the idea that the existing sensor could also be modified so rather than connecting the phototransistor to the tach's inputs, I use the buffer/etc circuitry. Perhaps just removing the Hall-effect sensor and connecting the phototransistor?I have recently discovered these 'optical switch' modules which feature fibre optic cables and have NPN output and even allow you to set sensitivity as well. They're DIN rail mount and require a separate +12v supply, but this will be available in the machine anyway.So I will poke around the tach's PCB to see if I can find an optoisolator or similar, or just trace where the input signal goes and work out what kind of signal it is expecting.QuoteI can do better than a shield: the sensor will be fully-enclosed in the cast iron shell of the milling machine.

author: daxliniere
author_rank: Regular Contributor
timestamp: March 29, 2025, 01:09:39 am
content: I got excited for a moment as I remembered that Hall-effect sensors can be triggered simply by gear teeth but a) the signal would require additional logic processing to divide by the number of teeth on the gears, b) one of the 2 output-shaft gears is non-metallic and c) I can't use just the steel gear because when it's not engaged it doesn't spin at the full output RPM.Oh well, back to plan A of using a photo method.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 29, 2025, 03:24:14 pm
content: As per the manafacturers diagram?

author: daxliniere
author_rank: Regular Contributor
timestamp: March 29, 2025, 08:51:49 pm
content: Quote from: Terry Bites on March 29, 2025, 03:24:14 pmPlease don't tell me this post is what it looks like.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 31, 2025, 05:54:47 pm
content: Surely an npn photo transistor with a pull-up fits right in?A ready made photo interuptor would be a good choice. A faff to illuminate and transitions might not be sharp enough.I'd hope that the input to the tach had some schmitt trigger action going on. You can test for that maybe.Hall sensors are the way ahead I'd say. They need a small bias magnet to push the sensor nearer to its detection threshold.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 01, 2025, 12:47:56 am
content: Quote from: daxliniere on March 29, 2025, 01:06:13 amA reflective sensor might not like a dirty environment.You can work back from the sensor datahttps://www.handsontec.com/dataspecs/sensor/NJK-5002C-Hall%20Sensor.pdfAn equivalent optical slot, logic out sensor could behttps://www.lcsc.com/product-detail/Photointerrupters-Slot-Type-Logic-Output_Saiya-Sensor-SY-LS670-16_C22373792.htmlYou could fix a stainless cable tie(s) to the axle and have it interrupt the slot. A slot sensor is a bit more tolerant to dirt and dust.

author: Poroit
author_rank: Frequent Contributor
timestamp: April 01, 2025, 06:02:54 am
content: Monarch has a range of Speed sensors which should be able to interface with the AC Tachometer.https://monarchinstrument.com/product-category/sensors/I have used ROS & ROLS successfully on projects.

author: daxliniere
author_rank: Regular Contributor
timestamp: April 01, 2025, 09:53:17 am
content: Thanks @PCB.Wiz. It's actually quite clean inside there as the only grease is captive in the bearings (each steel gear meshes with a Tufnol/phenolic gear).The cable tie is a really good idea. I will resort to that if I can't get my original IR-absorbing tape idea to work correctly with the fibre optical switch I bought. Hopefully its sensitivity control means I can tweak it to perfection.Will update once I receive the optical switch and have tested with it. In the mean time, I'm going to trace the input pin to see what I can learn about the input circuit.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: April 01, 2025, 01:59:33 pm
content: Good Morning Daxliniere, There are a lot of handheld tachometers on the market, even Amazon has a bunch:https://www.amazon.com/s?k=tachometer+hand+held&adgrpid=1334807683197444&hvadid=83425720688451&hvbmt=be&hvdev=c&hvlocphy=59456&hvnetw=o&hvqmt=e&hvtargid=kwd-83425832045393%3Aloc-190&hydadcr=977_1014987048&mcid=de48c46fa9fb33b4a3846265b0e493ef&msclkid=f7b9bd0e297515ec50addae5d96037a2&tag=mh0b-20&ref=pd_sl_ukio3rfgk_ePerhaps you could hack one to fit your situation