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Subject: Re: R-C circuit didn't work

Date: 08/09/02 at 6:43 AM
Posted by: David Ashby
E-mail: dashby@quin.co.uk
Message Posted:

In Reply to: R-C circuit didn't work posted by Mike on 08/06/02 at 8:20 PM:

Hello Mike,

I've been away for a couple of days, and have just seen that you've had a lot of trouble with the reset circuit. I didn't know you were going to drive an LED array as well as the RESET pin of the counter; as you found out, the R-C circuit will not pass enough current to power the LEDs, it can only drive small loads (such as the RESET pin, which is a high impedance CMOS input).

I see you've tried using a 2N3904 to buffer the reset signal - it's the right direction to go, but operation still seems unreliable. For one thing, the base current of the transistor will be high enough to shorten the reset time significantly, especially if the transistor is driving the LED array AND a 100 ohm resistor between the emitter and ground.

Here's another idea: do you have a spare schmitt trigger inverting gate on the board? (e.g., 1/6 of 74HC14?). If you have, connect the R-C network to the input. For an inverting gate, you'll need to down the network upside down - resistor to positive supply, capacitor to ground, diode cathode to positive supply, anode to inverter input). Then take the output of the schmitt trigger to

a) RESET input of the counter I.C.
b) Base of 2N3904 through a series resistor @ 1K.

I don't know why you have a 100 ohm load at the 2n3904 emitter in parallel with the LED array - it just forces the 2N3904 to supply additional current unnecessarily - so I would advise its removal unless it's there for some reason I'm not aware of.

David Ashby

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