Re: 555 monostable circuit design

John Fields wrote:
On Thu, 06 Apr 2006 13:08:25 -0500, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:


On 6 Apr 2006 07:46:27 -0700, jeff@xxxxxxxxxxxx wrote:


I have a GPS unit which I'm mounting in the trunk of my car powered by
the 12v lighter plug in the trunk. The socket has power only when the
key is on and the GPS unit requires its power button to be held down
for 1 second to activate it. If I were to use the 555 monostable
circuit I found (link below) to trigger a relay that would act as a
remote switch, would I be able to omit the circuit's pushbutton and
just use the lighter plug's power to start the countdown? Also, it
seems as though there is a 6 second delay built in to the circuit, what
component values would I need to change to get the 1 second delay I
need?

---
That circuit won't work for you because it needs a trigger to start
it after power has been applied. What you need is a circuit that
turns a relay on as soon as you turn the key on, then releases the
relay 1 second later.

However, that would be equivalent to applying power to the GPS at
the same time that its POWER button was held down, and it might not
like that.

If it didn't mind, though, here's the simplest way I can think of
solving your problem, plus you get a little delay between the
application of power to the GPS and connecting the PWR terminals
together (pressing the POWER) switch because of the switching time
of the relay. View in Courier.

VBAT>-------+
|
O--> |
IGN |
O | +---------+
+------------->>--|+12v |
|+ | |
[CAP] COM | |
| O---->>--|PWR |
[COIL]- - -| | |
| O--> | | |
| |NO | |
| +--------->>--|PWR |
| | |
GND>-------------+-----------------|GND |
+---------+
GPS


---
I decided to try it, and after some experimenting with an Aromat
JT1a-DC12V relay, this circuit gave me 1.1 second from ignition on
through relay on to relay off.

+12V>-------+
|
O--> |
IGN |
O |
+-------+
| |+ [1kR] [3300µF]
| |
+-------+
| | O---->
| [COIL]- - -| |K | O--> |
[1N4148] | |NO | | +--------->
| | GND>-----+-------+



Nice.
.