timing circuit (lm555)

[JO3]

Hello all,

I'm trying to make a circuit with the following properties:

1) output at +V for a time T1
2) output at 0 for a time T2
repeat indefinitely
**also, whenever contact is made between two points, I would like the circuit to jump to state 2 (contact would only be made when the system is in state 1)

It would be good if I can adjust T1 and T2 continuously, i.e. with a variable capacitor and/or potentiometer. T1 should be about 15 minutes and T2 should be about 45 minutes, but I want to understand the circuit enough to be able to make adjustments myself. Equations would be very helpful:)

I think I might be able to use the lm555 (http://www.national.com/ds/LM/LM555.pdf)or lm556 (http://www.jaycar.com.au/images_uploaded/LM556.PDF)chip, but I'm not exactly sure how to set these up. Voltage should be between 5V-15V or so.

Thanks for your help,
Joe

[dlgoff]

Here a google search for 555 timer circuits (http://www.google.com/search?q=%22555+timer+circuits%22&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a).
The first one looks like a good one.
http://www.uoguelph.ca/~antoon/gadgets/555/555.html

Welcome to PF

[JO3]

Thanks, that site looks very helpful. I'm reading through it now.

Joe

[berkeman]

It would be good if I can adjust T1 and T2 continuously, i.e. with a variable capacitor and/or potentiometer. T1 should be about 15 minutes and T2 should be about 45 minutes, but I want to understand the circuit enough to be able to make adjustments myself. Equations would be very helpful:)

Those are pretty long times to be trying to use a 555 analog timer circuit. A better solution would be to use a digital oscillator (based on a 32kHz watch crystal, for example), with divider circuits and a little logic to give you the variability you want. Check out the 74HC4060 IC and its app notes, for example.

[Averagesupernova]

WOW! I thought I was pushing the envelope timing out to around 10 minutes with a 555.

[Redbelly98]

I got one to go to 40 minutes. It's in a coffee-cup heater, the heater shuts off after the 555 says it's time.

The heater circuit originally timed out at 20 minutes. I opened it up, changed a 5M resistor to 10M, and now it's 40 minutes.

[dlgoff]

I've had them work for long times also. Nice big caps.
I think that the 555 is a good way to get your feet wet in circuit building.