Newbie project, mechanic needing delay timer & more i12v

[greg123]

Help making 12v delay timer & warning buzzer in switch box

Hi folks,

Thanks for taking the rime to read this. As a mechanic i'm ohm's law & soldering/faultfinding literate in 12v apps as well as familiar with basic descreet componants eg resistor led capacitor etc.

I need some help making a little controller box to mount on the dash. it will need to run on 9-15v dc (12v-14.4v nominal automotive) unsmoothed.

It needs a single pole double throw switch (3 terminal) @ 5a to control the output, 4 led's that are easy to mount on box.

Now here is the interesting bit, I need a delay timer such that when switch is moved from position 2 to position 1, a 5a output is fed for an adjustable time between 1& 60sec. At a push the output could be nominal & drive a relay elsewhere. The time would need to not easily be tampered with, so recessed pot or removable cover to accesss dip switch needed. Also needs to have a small buzzer that sounds when switch is in position 2 and an input 12v is killed, ideally on a 20sec timer but i can live without that for simplicity sake, purpose is to warn driver if he turns ign off without switching unit to position 1.

Having 2 blade fuses accessable in side of unit would also be nice, or maybe under part flip or sliding cover with pot or dip switches.

whole unit must be small as poss, to fit on dash by other switches by sticky pad, 40mm high x 20mm deep x 80mm long i had in my head. 5 to 7 wires of 5a will be connected.

I have seen adjustable 'delay on break' relays for £25, they wouldnt fit in small box so i'd need a two part unit with timer under the dash - not so easy to adjust & harder to fit. besides i am hoping to make the whole thing under £25. other than timer its just leds, box, wire, switch, buzzer.

I may need a few & may be interested in paying someone to assemble for uk use. getting the design & sourcing seem the hard bits.

Can anyone help point me in the right direction please?

Greg.

 

[Hero999]

For simple timeing you can use a monostable 555 timer circuit.

If you want anything more complex then a microconrtoller is probably your best bet.

 

[greg123]

Thanks for that. I just need to be able to set it up between 1 and 60 seconds and it needs to handle input voltage of 9-15v. IT will need to switch a 10a output but obviously I can use it's output to trigger a relay. Will I need to make a circuit board? I have no kit to do that I guess I can research it.

Greg.

For simple timeing you can use a monostable 555 timer circuit.

If you want anything more complex then a microconrtoller is probably your best bet.

 

[Hero999]

The delay can be varied from 1.1s to 67s.

The relay is already shown in the circuit will switch 10A providing you select one with the correct rating. Note that the 555 timer has a maximum output current of 200mA so don't choose a relay with a lower coil resistance than 75hm: or it'll cook the IC.

 

[greg123]

Perfect, thanks. I'm gradually understanding it. I'm now selecting componants on RS - any particular type of resistor and capacitors I need? I have searched the web and can't find out what wattage resistor I need, I presume .25 watt is okay?

Also I note on the diagram it's a 9v supply, the 555 I have picked is said to work at 15v, 18v max - so that should be okay for my 12v - 14.4v nominal car voltage, but would that mean the resistor/capacitor values are going to be different?

Appreciate the help, I have ordered up a breadboard and some fly leads so pretty soon I should be tinkering!

Greg.

 

[Hero999]

The resistor and capacitor values are the same regardless of the supply voltage.

0.25W resistors are fine. I tend to go for metal film myself, though carbon film will do fine.

TTL 555s will work from 4.5V to 15V, 16V maximum but some CMOS variaties will work up to 18V.