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I have a 555 pulse generator controlling a relay going a little faster than 1hz. It is hooked up to start up when it receives positive voltage from the switched accessory circuit of a vehicle. That much works fine, but I would like to limit the number of pulses it puts out to around 15 or so, and then stop until the next time the vehicle is switched back on. Using another 555 chip, I thought about making a "one-shot" pulse generator to control the first one, but it seems that this circuit requires a negative pulse to initiate it. That doesn't quite work for the automated status that I require. Any thoughts?

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  • \$\begingroup\$ You can convert a positive pulse to a negative one with a single transistor and a resistor. \$\endgroup\$
    – Hearth
    Feb 18, 2019 at 1:52
  • \$\begingroup\$ True, but it's not a positive pulse, per se. The car is started, so the positive remains positive until the car is turned off again, but the negative must be a short pulse to trigger the 555. \$\endgroup\$
    – Viii Cards
    Feb 18, 2019 at 2:34
  • \$\begingroup\$ I thought of a rather inelegant way to do it using two additional relays and a 1000uf capacitor, but I was really hoping to do it in a more streamlined fashion. \$\endgroup\$
    – Viii Cards
    Feb 18, 2019 at 2:48

2 Answers 2

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The cleanest solution would be to add a counter (there are still available 4-bit counters that can count 16 pulses). Simply have the counter clock off the existing 555 timer pulses. An R/C time circuit can provide a simple reset to assure that the counter resets to 0 each time the power first comes on. When the counter reaches full count use the output to reset the 555 chip so it does not produce any more pulses until the power cycle reset.

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  • \$\begingroup\$ I considered a decade counter, but I had just hoped there'd be an obvious way to do it with (2)555 chips. The necessity of the negative pulse to trigger the single-event timer which controls how long the pulse generator runs, has proven something of a quandary. \$\endgroup\$
    – Viii Cards
    Feb 18, 2019 at 3:45
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Here is a sample circuit how you can generate your pulse train with two 555 timers. The first 555 triggers just after the power comes on. When it triggers it takes the second 555 out of reset and allows it to generate the desired 15 or so pulses at about a 1 second rate.

Note that the diode D1 is used to give better control over selecting the on and off duty cycle of the repeating pulses rather than the normally seen configuration for an astable 555 circuit.

enter image description here

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  • \$\begingroup\$ Michael Karas - thanks for the schematic. Using a resistor and capacitor on the trigger was just the tip I needed. I actually POWER the second chip with the output of the first one, so I just have a jumper from pin 3 of 555(1) to pin 8 of 555(2) and it works like a charm. \$\endgroup\$
    – Viii Cards
    Feb 19, 2019 at 5:20
  • \$\begingroup\$ In fact, here it is: youtu.be/GddvjG5WyT8 \$\endgroup\$
    – Viii Cards
    Feb 19, 2019 at 5:21
  • \$\begingroup\$ vote it up if you like it, \$\endgroup\$ Feb 19, 2019 at 7:40
  • \$\begingroup\$ This is my first time here. How do I do that? \$\endgroup\$
    – Viii Cards
    Feb 19, 2019 at 18:48
  • \$\begingroup\$ Look at the arrow buttons near the upper left of the answer. A more comprehensive site usage can be read in the help section. \$\endgroup\$ Feb 19, 2019 at 20:23

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