timer

[Deigh]

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

 

[Arouse1973]

When you say placed on it what do you mean.
Thanks
Adam

 

[(*steve*)]

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.

 

[Deigh]

When you say placed on it what do you mean.
Thanks
Adam

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

 

[Deigh]

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.

Thanks but that is a bit to complex for me!

 

[Arouse1973]

Ignoring the physical and triggering issues (that your explicitly not asking us about)

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.

 

[(*steve*)]

What's that suppose to mean Steve.

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.

 

[BobK]

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

 

[Deigh]

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

 

[(*steve*)]

I wonder if you've purchased a kit that uses a 555

 

[KrisBlueNZ]

Hello Deigh.

I think we probably do need a bit more information.

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.

Give us some more information and we can give you some better advice.

 

[Deigh]

I wonder if you've purchased a kit that uses a 555

I havn't opened it yet but you can be sure that a 555 will be there!!!!

 

[Deigh]

Thanks for that rep

Hello Deigh.

I think we probably do need a bit more information.

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.

Give us some more information and we can give you some better advice.

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.