contactless frequecy measurement.  suggestions requested - Page 1
Forum section: electronics / projects

author: kyndal
author_rank: Regular Contributor
timestamp: June 29, 2016, 05:49:58 am
content: Hi Guys.i got a bit of a brainstorm going on.I have a ground / signal wire pair. with the "signal" being 5V frequency varying between 0 and say 200 hzfor an RPM gauge. no current to speak of.Any ideas for the best way to contact-less detect / count this signal ?I'm thinking about the contact-less voltage detectors / Hall effect sensors ? "antennas" ferite core/transformer etci have tried a few things. but not having much luckany suggestions as to the direction to look ?I'm hoping to build a standalone frequency display. without disturbing the system (cutting wires / piggybacking) etcThanksRune Kyndal

author: bktemp
author_rank: Super Contributor
timestamp: June 29, 2016, 06:12:07 am
content: Current transformers and hall sensors work only if there is current flowing. If it is only a signal line, then it will not work.You could try capacitive coupling: Add a wire next to the signal wire. If the signal is a square wave it sould work: The low coupling capacitance will form a highpass filter, giving a short pulse at every edge of the signal. This needs to be amplified close to the pickup point.

author: kyndal
author_rank: Regular Contributor
timestamp: June 29, 2016, 06:29:52 pm
content: thanks for the suggestion Bktemp.capacitive coupling... sadly I'm not good with analog stuff.. how i would go about doing capacitive couplingi understand that the insulation on the signal wires. and the two conductors form a small capacitor.but how do i go about getting the coupled signal amplified ?can i hook the "antenna?" wire directly to the input of an opamp ? or the base of a transistor ?thx

author: bktemp
author_rank: Super Contributor
timestamp: June 29, 2016, 07:07:36 pm
content: The coupling capacitance is probably very small (<1pF). Therefore you can only detect the transitions (if the signal has fast enough edges) and need to restore the steady state.I would try a comparator with a high input impedance. Maybe something like this could work:R1+R3 create a reference voltage. R4 biases the sense antenna wire C2 to the same voltage. R2 adds a bit of hysteresis to hold the state until the next transition.U1 could be a comparator with CMOS inputs (TLC372 with pullup at the output) or a opamp (TLC272).You probably need to adjust R4 and R2 and try different sense antenna wires to get the circuit working.If you have a scope you can mount the sense wire and measure if there are short spikes at each transition and optimize the sense wire.

author: kyndal
author_rank: Regular Contributor
timestamp: June 29, 2016, 07:56:09 pm
content: thanks alot bktemp.i do have a scope.. so i will give you idea a bit of a play around.see what opamps i got in the binthxRune

author: Berni
author_rank: Super Contributor
timestamp: June 30, 2016, 05:23:20 am
content: Twist another wire around a few cm the signal wire(not gound) and look at it with your scope. If the signal has sufficiently fast edges you should see a spiky signal on the screen where the transition happens.If that doesn't get enough coupling you can try wrapping a few cm of the signal wire in aluminum foil and connecting your scope to that.

author: CJay
author_rank: Super Contributor
timestamp: June 30, 2016, 07:17:40 am
content: Quote from: Berni on June 30, 2016, 05:23:20 amBit of copper foil, around 0.5cm2 worked wonders for collecting data on ignition systems from HT leads, it may or may not work for the OP

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2016, 01:42:35 pm
content: Use a couple of CMOS logic gates: CD4069, 74HC04 etc.Wrap the input wire round one of the conductors in the twisted pair.The output will be a clean square wave, with the signal going high on the rising edge and low on the falling edge.You may need to experiment with the value of Rf.

author: bktemp
author_rank: Super Contributor
timestamp: June 30, 2016, 01:59:04 pm
content: Quote from: Hero999 on June 30, 2016, 01:42:35 pmThis is exactly the other circuit variant I simulated yesterday:It is easier but I found it to be less sensitive because of the large hysteresis: The coupled voltage spike must reach at least Vcc/2, because the input will be biased at 0V or Vcc and must cross Vcc/2 to toggle the state.If the coupling capacitance is large enough it works, otherwise additional resistors are needed to set the DC bias at the input closer to Vcc/2 similar to my comparator circuit.

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2016, 02:51:02 pm
content: Quote from: bktemp on June 30, 2016, 01:59:04 pmAdding another amplifier stage will increase the sensitivity. The disadvantage of this would be CMOS gates draw a lot of current when biased in the linear region. To overcome this the CD4007 could be used with resistors added to reduce the current consumption of the linear stage.

author: kyndal
author_rank: Regular Contributor
timestamp: July 01, 2016, 05:54:46 pm
content: thanks for the input guys.got a few things to play around with./R_Kyndal