violet

Please..any idea to count 15minutes by using simple electrical circuit. (simple RLC circuit??) Actually i need to switch on the circuit after 15minutes. But how to count the 15minutes?? any idea? :roll:

Nigel Goodwin

A 555 timer would do it, although it's accuracy won't be great.

Have a look at http://www.uoguelph.ca/~antoon/gadgets/555/555.html for help with 555's - figure 8 is a 10 minute timer, you could easily make it 15 minutes.

If you do a google search for '555 timer tutorial' there's a lot of info out there.

Sebi

I prefer the CD 4541 programmable timer...

Nigel Goodwin

I prefer a PIC :lol: but I was trying not to suggest that!.

ljcox

The simplest solution is to use a CMOS Schmitt Trigger such as the 74C14, 40106, etc. Connect a resistor to the input, and a capacitor from the input to gnd.

This will delay the signal applied to the resistor (ie. a step function) by about one time constant, ie. R * C

It won't be very accurate, but it is cheap. You will have to adjust R and/or C on test to obtain the delay you want.

Also, the "off" delay will be about the same length. To reduce the "off" delay you have to speed up the discharge of the capacitor. One way to do this is to connect a resistor in series with a diode and connect them in parallel with the resistor.

Len