Should coupling capicitors be used at inputs to BS2 ?

po2le

I'm using the BS2 to count pulses from a motorcycle ignition.· The pulses·come at·low frequencies, usually·less than 100pps.· The pulses are rather ragged looking, as they come from the low side of the ignition coil (transformer).· I run the pulses thru a resistive voltage divider, which drops the pulse volts to less than four volts.· From the divider, the pulses are applied to a Schmitt trigger (home-made from an·IC·555 timer), which does a fair job of cleaning up the pulses, but not real good.· The pulses are somewhat square, and not evenly distibuted over time. The Schmitt does have a coupling cap at the input, and I don't know if that is needed; maybe its distorting the pulses.·
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However, should I use coupling caps at the input to the bs2 ?

PO'T

Mike Green

Why? Do you understand the purpose of coupling capacitors in logic circuitry? If you don't understand something, don't just stick it in a circuit or, if it's already there because it's in someone's suggested circuit, don't just remove it. Find out why it's there and what purpose it serves first.

To more directly answer your questions, you need to supply more information. In particular, what's your existing circuit? What do the pulses look like? How are you powering the BS2?

po2le

Mr. Green,
I was first introduced to electronics while in the US Army, back in the day when they still taught electron tube circuitry as a basic course. I remember what coupling caps were used for and how they operated, at least as they applied to amplifiers and pulse producing circuitry (of that time). Now, years later, I'm an old retired guy with an interest in electical/electronic circuitry, working on a hobby and trying to learn some new things. I just wanted some advice on whether or not coupling caps should be used with this type of circuitry.

My circuitry: Basic Stamp homework board with Embedded Basic stamp 2, powered by 9v battery. It has a serial port connector, a header for connecting i/o pins to embedded breadboard, with volt regulator and some EEPROM. I'm taking pulses from a motorcycle electronic ignition at the primary side of the ignition coil (the same point that the tach. circuit is fed from). These pulses are not quite square, there is a lot of noise involved (there seems to be a neg and pos portion to the pulses, one part is quite square, while the other is a raggedly shaped sawtooth). After dropping the amplitude of the pulses, they are fed into a 555 Schmitt trigger, which is powered by 5v dc. The pulses are coupled into the 555 by a .01 ufd cap thru a resistor to gnd. That resistor is also part of a volt dividor composed of 2-100k resistors from VCC (5v) to gnd. The output of the 555 is then fed to pin 0 of my bs2, where I use the 'count' command to count pulses, and the data (pulse count) is saved in EEPROM. IT WORKS.

Beau Schwabe

po2le,
·
In your particular application, a 555 Schmitt trigger configuration may not be the best choice.· I would intuitively lean more toward a 555 Pulse stretcher (aka: Bounce free switch) for this application.
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The coupling cap on the front end of your current Schmitt configuration is most likely to offset any DC bias there might be.· It would probably be a good idea to leave it there using a 555 Schmitt trigger, 555 Pulse stretcher, or any other configuration.
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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 6/15/2008 6:38:36 AM GMT

Mike Green

Thanks for the information. It sounds like the Schmitt trigger is working for you. I found a description with a websearch of this Schmitt trigger here www.uoguelph.ca/~antoon/gadgets/555/555.html. One thing they warn about is the need for adequate power supply bypassing because of the high current surge possible. You should have a 0.1uF capacitor at the 555 supply pins as close as possible. The Homework Board has a 10uF capacitor on the output of the regulator, so you wouldn't need another one of those.

Coupling capacitors in logic circuits provide a differential function on the input signal. Essentially, they take a pulse and convert the leading and trailing edges of the pulse into brief pulses, one positive and one negative. Often these are used as multipliers since you get two pulses for one. The reset circuitry for the Homework Board uses this to make sure that toggling the DTR line on the programming port causes only a brief reset pulse for the Stamp even when the PC leaves the DTR line active as long as the serial port is open.

po2le

Thanks to Mike and Beau. I appreciate the information.
Mike: Yes, the Schmitt trigger in that location (www.uoguelph.ca/~antoon/gadgets/555/555.ht), that you suggested in your reply, is thecircuit that I made and used