A timer question

Hi, Tom. I like the CMOS 4538. It has two one-shots on one 16-pin IC Each has separate input pins for positive edge triggering and negative edge triggering. Each also has a reset input, and positive and inverted outputs. Output pulse widths extend from less than 100us to over one second. Timing is set with one R and one C per one shot.

For pulses less than 100us, use the 4528, which has the same pinout.

Both operate on +3V to +15V supplies, like the 555. The only negative is that the output is made to drive CMOS logic gate loads instead of the high load a 555 can drive. However, by adding a transistor and two resistors, you can make the 4538 drive as much or more current than a

Also, the 4538 is set up to be retriggerable (another pulse will restart the time). However, it's trivial to wire it up to be non-retriggerable, like a 555.

Good luck Chris

I am assuming you want a pulse to be generated any time a signal changes state. If you connect the signal to an exclusive or gate, and also connect a delayed version of the same signal, the gate will output a negative pulse equal in duration to the signal delay. You can use the three extra gates in the pack to produce the delay. If you add an RC to the delay, you can lengthen the pulse as needed, possibly eliminating the 555. If you reverse the polarity of one of the delaying gates (by switching the unused input from a low to a high) the circuit will output a positive pulse.

I suggest either a 74HC86 or a 4030 or 4070.

Have you also considered using a microcontroller like PIC ? It is a little investment on the programmer but is a good long term investment.

You can trigger your 555 with the negative pulse, through a diode. You can run the positive pulse through an NPN transistor inverter, and run the output of that through another diode to the 555.

Input o----+---||---+---/\\/\\/\\---o Vcc | D2 | | | | c/ +-----/\\/\\/\\-----| Q1 R1 e\\ | Gnd

R1 = 1K; R2 = 470; Q1 = 2N3904

Hope this helps. Ed

Hi,

Thanks a lot for all your help! I really appreciate.

Tom