# timer

Discussion in 'General Electronics Discussion' started by Deigh, Apr 8, 2014.

1. ### Deigh

171
8
Apr 26, 2011
I am involved in a project where a small and inexpensive timer is required. Can anyone help me with a circuit?
Requirements are that it be battery powered and that anything placed on it would cause a light (LED) to switch on and stay on for a certain time. This time should be adjustable from (say) 20 seconds to 60 seconds.
Ideal finished size of unit would be like the dimensions of a matchbox, and the object to be placed on it would be like a tablespoon.
If there is something already on the market I'd like to know about it.
Regards to all.
Deigh Davies

5,164
1,087
Dec 18, 2013
When you say placed on it what do you mean.
Thanks

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
Ignoring the physical and triggering issues (that your explicitly not asking us about) it seems your solution is a non-retriggerable monostable. Googling should reveal many circuits. A555 circuit is the obvious choice, although 1 minute is getting into the twighlight zone where the real and calculated times may have significantly diverged.

The non-retriggerable part is generally achieved by placing a capacitor in series with the triggering source. Whilst this does not exactly make it non-retriggerable, it means that a continuous trigger signal does not hold the device in a likewise fixed state.

4. ### Deigh

171
8
Apr 26, 2011
OK, so you have a spoon in your hand and you put it down on the unit and take your hand away. Sorry if that sounds patronising but it was the only way of explaining it.

Deigh

5. ### Deigh

171
8
Apr 26, 2011
Thanks but that is a bit to complex for me!

5,164
1,087
Dec 18, 2013
What's that suppose to mean Steve. He want's a simple circuit to light an LED so I guess understanding the whole project is the best place to start.

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
Well, I was certainly not suggesting they should be ignored...

But who knows, perhaps he has everything worked out and just needs a simple three terminal thronomister to do the timing. You'll note that I described a very complex circuit, carefully ignoring the simple one-component solution that can be made from a capacitor and a piece of string.

8. ### BobK

7,682
1,688
Jan 5, 2010
Yes, you can make a timer from a capacitor and a string. If the string is length l, the period is 2*pi*sqrt(l / g). The capacitance and voltage rating do not matter, though a larger weight is helpful.

Bob

9. ### Deigh

171
8
Apr 26, 2011
Sorry, this has got a bit out of hand. I was hoping for a simple circuit and all I got was complicated questions and answers. Went into a parts store yesterday and bought myself a cheapo kit. well, it wasn't all that cheapo! in fact was darn expensive for what it contained but it will set me off with something I can experiment with.

Will come back again when I've got something going to ask for advice on improvements.

Thanks anyway!

Dwigh

10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,501
2,841
Jan 21, 2010
I wonder if you've purchased a kit that uses a 555

11. ### KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
Hello Deigh.

You want the unit to be battery powered, and to fit into a matchbox?

Have you worked out how you will detect when something is placed on it? A very sensitive switch perhaps?

Can you describe the purpose of this thing? That might help us suggest the most appropriate solution.

I was going to suggest something powered from a CR2032 3V lithium button cell, with a high-efficiency LED that will glow moderately brightly at 1~2 mA so the cell would last a reasonable amount of time. The LED I suggest is the HLMP-D150 (see http://www.digikey.com/product-detail/en/HLMP-D150/516-1323-ND/637587). It's a 5 mm diameter red LED with a diffused lens.

12. ### Deigh

171
8
Apr 26, 2011
I havn't opened it yet but you can be sure that a 555 will be there!!!!

13. ### Deigh

171
8
Apr 26, 2011
Thanks for that rep

Thanks for that reply. will make sure next time that I give a lot more details when I ask a question.
When product is working I will be wanting to make it in quantity so will be back asking for cheap alternatives for the bits.