555 one shot

KatyBri

I have a STAMP project where one of the inputs uses a 555, configured as a one-shot. When the 555 is powered, its output pin goes high for 5 sec.

I am ambarrased to say I can not make it work. I want the 555 to simply turn on its output for 5 sec. when its power is applied. I have searched the web and tried many circuits, but none work. They keep the trigger open and require it going o ground to initiate the pulse. I don't want to deal with the trigger or reset pin, just apply power to the 555 and have it output one 5 sec. hhigh pulse.

Any elp would be appreciated.

kwinn

You need a large enough capacitor between ground and the trigger pin to keep the voltage below 1/3 of Vcc until the Vcc voltage stabilizes. Try something around 1-2uF to start with and a 1Meg pullup resistor from the capacitor/trigger pin to Vcc.


PS ope it elps.

stamptrol

If you only need a one-shot, the 74HC123 is a good candidate. Simple to use, retriggerable and 2 in a package.

Cheers,

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Tom Sisk

http://www.siskconsult.com
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Beau Schwabe

KatyBri,

You really don't even need a 555 or 74HC123 for this ... just a PNP transistor, a couple of resistors and a capacitor.

I know you said that you didn't want to mess with a trigger, but if you look at the 'Trigger' in the schematic as if it were your power to a 555 instead, I think this circuit will work for you.

When the 'Trigger is LOW or left floating' the 100uF cap will charge very close to 5V very quickly and draw a minimal amount of 'standby current' once the capacitor is charged (127 micro Amps) ... when you apply 'power' or place 5V on the Trigger, the transistor turns OFF, and the Capacitor discharges through the 39K resistor. It takes approximately 5 seconds for the voltage across the capacitor to reach 1.4V in this setup. Since the threshold of the STAMP is about 1.4V the STAMP will 'see' this as a LOW only when the capacitor falls to this level or below.

If you need to make adjustments to the time, you can substitute the 39K resistor with a variable 100K Pot.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.

Post Edited (Beau Schwabe (Parallax)) : 1/11/2010 11:28:27 PM GMT

KatyBri

Thanks to everyone for your suggestions. I now have a working circuit.