Small timer 'project'

[Harryg]

I hope someone can help me with this project.

I need a very small switch device, no bigger than about 15 mm in diameter and up to say 25 mm thick, and powered by a small watch battery. The device would have one main function - when you press the switch, a light (say LED) flashes, at say 1 to 2 sec intervals for around 30 mins, then goes off. That's it. It would also be nice if it had a battery low function so that the LED would say flicker continuously for say 15 seconds if the battery was below a certain useable level.

Many thanks

Harry

 

[Dragon Tamer]

You're asking an awful lot from us chickens. The easiest way to do this would to be using a 555 timer. It needs no programing other than the RC network it uses for timing and it can be made fairly small. The down side is that 3V is pushing it's lower voltage limit and it will be very hard to get it ranged in at exactly 30 minutes.

 

[Harryg]

Thanks Dragon Tamer - You can see that I am clearly not an eletconics expert! On the postive size, the only specification that is tight is the size. The durtaion can be anything around the 30 min mark, and can have a reasonable tolerance each time the switch is pushed. The object of the device is that if the light is flashing then around 30 mins have NOT elapsed since the switch was pushed.

 

[Dragon Tamer]

What you need are 2 different timers, both can be 555 timers or they can be a single 556 timer (just two 555s in one package). One needs to be monostable (it only triggers once) and one needs to be astable (triggers repeatedly).

Here is the schematic for a monostable 555:


The output of the monostable (pin 3) would trigger the astable 555 on pin 4 (the reset pin). When pin 4 on the 555 is held low the output will be low so the LED will not blink. Here is the schematic for an astable 555 timer:


All you need to do is to calculate the values for the RC network. Luckily this website has a tool that can tell you what frequency the output of the 555 will be based on the values you used, the catch is that it only works for the 555 in astable mode. For monostable mode you will need to figure the values out for yourself. The amount of time will be 1.1 x R x C, you can also flip this around to solve for R or C based on time. t (in seconds) / 1.1 / R or C.

At longer time constants the reliability of the 555 becomes severly compromised, 30 minutes is pushing that limit quite hard. You may need to use a counter, something like a 4017 could count 10 inputs, if each input lasts only 3 minutes (3 minutes is a much better time constant for the 555) then you end up with 30 minutes out. The only thing with that is that while the 4017 is set to trigger the astable mode, it needs a way to either retrigger the monostable 555 or the 3 minute timer would need to be a 3 minute astable timer that is disabled when the other astable 555 is disabled.

 

[vsaar]

It could also be done with 555 astable timer, 4040 binary counter and 4013 flip-flop. Put a switch to the input of the flip-flop, and the output to the trigger pin of 555. When you push the switch, the flip-flop output goes high and the 555 starts oscillating. Set the 555 to output about 2Hz signal into the clock input of 4040. The first output stage of 4040 is now giving a signal of 1Hz, which can be used to drive a transistor which then drives the LED. Every further output will be at half speed of the previous one, so the 12th output stage of 4040 will go high after 2048 pulses (about 34 minutes with 1Hz signal), and that pin must be routed to the reset pin of the 4040, and to the flip-flop to turn off the 555.

However, this will likely not fit into such a small space you described. :\

 

[KMoffett]

So, one of the critical factors in what color LED you want to use. White LEDs' operating voltage is often higher than the 3V terminal voltage watch batteries. I would use a red, green, or yellow LED, a PICAXE 08M2 micro-controller, five 1/8W resistors, a 1N4148 diode, a 3V lithium button battery (or two 1.5V buttons depending on shapes), and a miniature tactile PB switch. This would occupy ~1 cc volume, excluding the battery and switch. How many 30 minute cycles do you expect this to operate, and over how long a time?

It might help if you elaborated what is the goal of your project. Sometimes it's easier to help solve your problem, than help solve your solution.

Ken

 

[alec_t]

I think a micro is essential here, as per KMoffett's suggestion, in view of the space constraints. Using 555s would require a fairly chunky very low leakage capacitor for a 30 min period.

 

[MrDEB]

You people give up to easy!!
Look at a 7555 or a 7556
7555=60ua current and the 7556 = 120ua
2-18v range
Might be of some help??

 

[MrDEB]

As far as chunky caps go with SMD

 

[MrDEB]

other options
**broken link removed**
or
use a 7555 and a cd4017 decade counter

 

[MrDEB]

another option is look at using a cd4060
do a search as there are LOTS of LONG timers using this.

 

[KMoffett]

Typical. No response from the OP in almost a month. My guess is that this is the same (almost) project over on:https://www.edaboard.com/thread235218.html#post1006035

Ken

 

[MrDEB]

Your probably right KMoffett
Can't tell you how often I see this sort of thing.

 

[KeepItSimpleStupid]

BTW: Look here: **broken link removed**

There was an article on "pill-sized" timers. i.e. tiny.