# Reading Pulsewidth

Discussion in 'General Electronics Discussion' started by microchipdip1, Dec 20, 2010.

1. ### microchipdip1

2
0
Dec 20, 2010
Hi,

I am new to the forums. I am trying to read a signal with a microchip. However the microchip has a max input of 5v. The incoming signal is a square wave (0v, 12v). At the end of a hi pulse, it spikes to 40volts or even more. Id really appreciate it if someone could tell me what components I would need to put in that circuit to drop it down to (0v, 5v), and block out that spike.

Thanks,
Steve

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,480
2,828
Jan 21, 2010
Try something like this:

The 10K resistor limits the current. The 2 diodes clip any voltage higher than 5 or lower than 0) The additional 1k resistor limits the current through the protection diodes on the microchip should a really large spike come along,

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3. ### microchipdip1

2
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Dec 20, 2010
Can you point me to the most suitable and preferably cheap diodes that will do this? I am inept with electrical components. I only know how to program the microchip. Would TVS diodes be suitable?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,480
2,828
Jan 21, 2010
Almost any diode will work in that circuit. The only exception would be zener diodes rated at significantly less than 5V -- in fact, making the bottom one a 5.1V zener has some advantages (but I digress)

1N400x would be fine (use whatever is cheapest). If you have some 1N4148's or 1N914's or almost anything.

A 40V spike will give you about 4mA through the diode. The reverse voltage will never exceed about 5V. Switching speed may be an issue, so I might prefer the small signal diodes if the spikes are especially fast (rise time) or high in energy.

Where is this input coming from (it's not the ignition coil of a car is it?)

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