Re: Simple delay circuit


domwells27@xxxxxxxxx wrote:
ive found this circuit:
http://img327.imageshack.us/img327/5206/r844gs.gif

Would that do the trick? I assume i attach a 9v battery on pins 8 and 1
of the 555.

How would i connect the alarm to pin 3? Also do i need to connect
anything to pin 2

thanks for any help

cheers, dom

Hi, Dom. One easy way to get additional use out of the 555 is to
include other things in the circuit -- like your switch, for example,
and a relay. Assuming you've got a power supply between 5 and 15VDC,
you can get hold of a relay with a coil of the same voltage that uses
less than 100mA and you could try something like this (view in fixed
font or M$ Notepad):

|
| SW1
| VCC-o
| __--o----o----------o----------o-----o------o---.
| GND-o | | | | | |
| | | | | | |
| 33K.-. | | | | |
| | | | | | | |
| | | | | | | |
| '-' | | | | |
| | | .---o-----o--. | |
| o----------)------o2 8 4 | D| C|
| .001uF | | | | - C|
| --- | | | ^ C|
| --- .-----o | | | | RY1
| | | | | 3o---o---. .---o
| === | | | | |
| GND | .-. | | |
| - R | | | 555 | -----
| D ^ | | .--o6 | -----
| | '-' | | | |
| | | | | | |
| '-----o---o--o | '---o
| C | |7 |
| --- | |
| --- | |
| | | |
| === | |
| GND | 1 5 |
| '---o----o---'
| |
| ===
| GND
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

The 555 can source or sink current (allegedly up to 200mA, but I'd keep
it to 50mA -- the output voltage bogs way down when you go over that).
This trick will ensure an easy, reliable delay-on relay contact for
your alarm you can wire up just like a switch. Here's how it works:

When your switch makes contact, the circuit is powered up. The 555 is
instantly triggered by the 33K resistor and the .001uf cap with a short
pulse as the cap charges up. The output of the 555 (pin 3) also goes
to Vcc instantaneously (in relay time). This keeps the relay from
turning on, because the other end of the coil is also connected to your
voltage. After the time period you choose by your R and C (68K and
100uF will give you about 8 seconds), the 555 output will go low. This
will apply voltage across the relay coil, so it will turn on. You can
wire the relay contacts up to your alarm just like a switch, so your
alarm will go on after the 8 seconds. It will stay on until you remove
the power, because the 555 will only trigger when the trigger pin (2)
goes low. That won't happen as long as there's power.

Note that when you turn the Single Pole Double Throw switch (SPDT) off,
the power connection is grounded. That removes voltage from the relay,
and discharges the timer cap C through the diode (both Ds can be
1N4001). If you have a SPST (Single Pole Single Throw) switch, just
load Vcc down by putting an extra 1K resistor from pin 8 (Vcc) to pin 1
(GND) to help discharge things when the switch opens.

I hope this has been of help. If you've already got the 555, and you
can scrounge a small relay, you should have your alarm working right
away. If you have any more questions, feel free to ask.

Good luck with your project
Chris

.

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