Trigger Monostable High not Low

The 555 circuit is almost what I'm looking for, except for a couple of things:

1) How would I modify the circuit so that the trigger is Vcc, rather than GND?

2) I need the output to switch a small relay - would I need a diode across the coil to prevent back emf?

Thanks in advance,
Chris

Posted by chrisall on 04/08/2007, at 13:57 GMT

I forgot to add .....

I forgot to add that the trigger is not a pulse - so basically the operation should be that when Vcc is applied to the circuit the relay is energised for say 30 seconds & is then de-energised, even though Vcc is still present.

Cheers,
Chrisall

Posted by chrisall on 04/08/2007, at 14:17 GMT

Trigger Monostable

By the addition of a simple npn transistor inverter circuit on pin 2 the triggering of the 555 monostable can be converted to +Vcc. The collector connection on the transistor is connected to pin 2 as well as a 10k resistor to +vcc. The emitter connection goes to the 0v ground rail and the base connection is the input. Triggering upon application of power can be carried out with a 10k resistor and a 100n capacitor network between the supply rails, the junction of the resistor/capacitor connected to pin 2.

Posted by Robert Neill (varient412@yahoo.co.uk) on 11/03/2010, at 22:11 GMT