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Tachometer Input

I have a device which requires a signal for a tachometer. We have this device in the training room for showing / teaching operation to mechanics.

The device gets it's normal input from a Magnetic Pickup (Inductive pickup) used on the flywheel. It goes thru a series diode then then passes by a 5 volt Zener, then goes into a microcontroller and displays RPM as well as other functions.

I need to make a simulator which I can adjust from 0hz to 5khz to work on the above. Currently we are using a gear on a drill press, which works, but we would like a small electronic design. I was thinking about a 555 type circuit, but a 555 puts out a digital signal and the mag pickup outputs AC. Does anyone have any simple ideas or schematics I could use for this?

Thanks.

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Reply to
Richard
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You might be able to use a diode between the common of the pickup and the common of the 555 circuit to 'trick' the pickup to think it is seeing an AC signal. The diode would raise the common of the pickup to a voltage potential that is higher than the 555 circuit.

Just a thought.

Reply to
Rileyesi

Richard, I would try first to just use the output of the 7555 (an improved C-Mos version of the 555) with a supply of 5V the output I would capacitivly (100uF) couple to the meter . As you have described it, the unit apparently only triggers on the 0-crossings, so the waveform is probably irrelevant. If this doesn't work you either have to build a real sine generator, or filter the 7555 output with a tracking lowpass filter. But 0Hz are not attainable. A simple sine generator can be made with a pic with filtered PWM output, this can also make very low frequencies (or DC), which require huge(infinite) capacitors when done analog. .

You're welcome

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ciao Ban
Bordighera, Italy
Reply to
Ban

apparently

I think this should work directly off the 7555 because on my scope, I have the frequency output pin used for calibration which outputs a 2khz signal which looks like 5vdc square wave (if I remember correctly it was 5 volts) and the tachometer did indeed pickup the signal,

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Reply to
Richard

Yes, I'd certainly have expected it to. Are you confirming your problem is now solved, and that you have a 555 astable connected directly to your Tachometer?

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Terry Pinnell
Hobbyist, West Sussex, UK
Reply to
Terry Pinnell

No, it's not solved yet because I haven't built the 555 circuit yet. I just sent the cal. freq. from my scope to it to see if it would read a digital 5 volt pulse.

have

volts)

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Reply to
Richard

On Saturday 09 October 2004 04:14 pm, "Richard" top-posted:

Well, build the 555 circuit, and just put it to the same place you put your 5V. pulse. Power the 555 from 5 volts, and you're done. (you did say that it did count the 5V pulse, right?)

Cheers! Rich

Reply to
Rich Grise

Built a 555 circuit and tested it by hooking it up to my scope. The 555 is working fine however I do not get a RPM reading on the device. So I gues it is expecting a zero crossing.

BTW, my calibration frequency on the scope was 0.5vpp and it does not trigger the device consistently either.

Anymore ideas?

Richard

was

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Reply to
Richard

Put the capacitor between the input and the timer, like I had described in my first post, this will shift the DC-level, so the input signal goes below

0, even when the timer is fed from only +5V. 
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ciao Ban
Bordighera, Italy
Reply to
Ban

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