Timer circuit

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waddles

Gawd
Joined
Oct 2, 2003
Messages
622
I need help finding a schematic for a circuit I'm trying to build. I need a circuit that will, when activated, wait X amount of time (based on a resistor/pot) and then activate a switch or relay or something of that nature. Basically: flip a switch, wait X time, activate something else (an LED in this case.)

I found this page that has a lot of things, but I'm not sure exactly which one is what I need.

Any comments/advice/links are appreciated, thanks in advance. :)
 
use a 555, those things are extremely useful for timers. you shouldnt even be using anything but a 555.
 
What kind of stores would sell this? I mean, I don't think I've ever seen anything like this in RatShack...

Will I have to track down an electronics hobby type store?
 
radioshack dont sell em, they sell the chip though if you want to build it your self. if you do, this could help
this is from a google search, and they will all work
http://store.qkits.com/moreinfo.cfm/QK85
http://www.quasarelectronics.com/3152.htm
http://www.hobbytron.net/R-UT-5.html
http://electronickits.com/kit/complete/elec/ck1611.htm
http://www.maplin.co.uk/Module.aspx...source=15&WorldID=&doy=28m4&Cache=24757,14156

search, use your imagination, ect... building your own isnt hard, but the kits come with PCBs and all parts, for cheaper then you can get the parts sepratly. but like i said, you can get the kits anywhere.
 
Ski Me said:
I think this is what your looking for.
timer
Click to expand...
no, i don’t think so...

  • first, that site is in the UK, which means massive shipping costs... this site says "$9 minimum" to us
  • second, the price on that page is in GBD. that means it costs around $32 American, plus an extra " US$7.50 to cover currency conversion"
  • third, its designed to switch 240v mains lines... according to
    waddles said:
    activate something else (an LED in this case.)
    Click to expand...
    he is looking to switch an LED, not high current AC voltage (meaning a hefty, slow, loud, ect… relay)
so i would stick with a simple 555 circuit, either home brew on a prefboard or a $10 kit from somewhere... closer...

next time Ski, if your going to post, you might as well actually read the thread and not just post the first thing that came up in a google search. nice try though :cool:
 
theshadow27 said:
no, i don’t think so...

  • first, that site is in the UK, which means massive shipping costs... this site says "$9 minimum" to us
  • second, the price on that page is in GBD. that means it costs around $32 American, plus an extra " US$7.50 to cover currency conversion"
  • third, its designed to switch 240v mains lines... according to

    he is looking to switch an LED, not high current AC voltage (meaning a hefty, slow, loud, ect… relay)
so i would stick with a simple 555 circuit, either home brew on a prefboard or a $10 kit from somewhere... closer...

next time Ski, if your going to post, you might as well actually read the thread and not just post the first thing that came up in a google search. nice try though :cool:
Click to expand...

Hey screwbrain total cost for unit and shipping is $21.95 and if you could understand specs when you read them it can switch up to 240VAC at 5A it's not a slow hefty loud relay thats just designed to switch high voltage. And I never use google for anything cause I know what works and what doesn't. But hey I've been working with electronics for about 20 years so don't listen to me go ahead and get your 555 IC solder it up then when it doesn't work spend another $20 for a premade circuit.
 
Aight...prepare for a bitch fight!!!

Anyways, a 555 will do exactly what you want to do. From online documentation, read about the astable configuration. This will basically allow you to set a duty cycle other than 50%. So you can flip a switch, set the time until the led turns on, and then set the time it stays on. If you need something more elaborate, you're gonna have to use an MCU.

As far as using a relay, completely unnecessary. The 555 outputs 5V and with the proper resistor, you can directly power an LED. You may have to add a transistor if the LED is drawing too much power.

If you can find a prebuilt circuit/kit that does EXACTLY what you want to do, go with that. If you have never worked with electronics before, a 555 may be a little beyond your scope. There are certain tools that are imperative, ie an ocsilliscope. Without the right tools, you have no choice but to use a prebuilt circuit.
 
....no bitch fight here... ;)

i dont think you need an osiliscope to work with 555's, a DMM and a decent app note an youll be alright
 
theshadow27 said:
....no bitch fight here... ;)

i dont think you need an osiliscope to work with 555's, a DMM and a decent app note an youll be alright
Click to expand...

I've never tried to use a DMM to test the frequency of non-periodic signals, but I'm pretty sure that wouldn't work. Frequency is the most crucial aspect to any 555 timer, and I seriously doubt the DMM can accurately reproduce frequencies of signals with a duty cycle other than 50% (astable mode).
 
he wants it to stay on for sevral seconds... its not realy a frequency, unless you messure it in a fraction of a hertz...
 
joecool234 said:
Aight...prepare for a bitch fight!!!

Anyways, a 555 will do exactly what you want to do. From online documentation, read about the astable configuration. This will basically allow you to set a duty cycle other than 50%. So you can flip a switch, set the time until the led turns on, and then set the time it stays on. If you need something more elaborate, you're gonna have to use an MCU.

As far as using a relay, completely unnecessary. The 555 outputs 5V and with the proper resistor, you can directly power an LED. You may have to add a transistor if the LED is drawing too much power.

If you can find a prebuilt circuit/kit that does EXACTLY what you want to do, go with that. If you have never worked with electronics before, a 555 may be a little beyond your scope. There are certain tools that are imperative, ie an ocsilliscope. Without the right tools, you have no choice but to use a prebuilt circuit.
Click to expand...
Hell yeah this guys know what he's talking about. If you want to take the time and effort to build a circuit for the sake of building a circuit then go ahead but if you want a circuit that does what you it to do then buy one thats prebuilt. I've never found one that outputs voltage from a connector off the board. Mostly I just had to solder right to the pin on the chip. I still think the timer I offered would work for his use the only other thing he would have to get is a power supply.
 
theshadow27 said:
he wants it to stay on for sevral seconds... its not realy a frequency, unless you messure it in a fraction of a hertz...
Click to expand...

I'm not exactly sure what the "proper" term is for it...but I, and everyone at work, refer to the overall period as a frequency. Isn't frequency just the inverse of period... But that's neither here not there. I think we scared the OP away anyways.
 
There is an even easier solution that hasn't been mentioned yet, a simple RC (resistor and capacitor) timer. Incidentally, an RC timer is the basis of the 555, but if all you need to do is turn on an LED after a switch is flipped, you don't really need a 555, unless of course you want the LED to blink.

Here is one simple RC timer circuit:
rctimer.gif


The idea is that as the capacitor charges, eventually it will overcome the breakdown voltage of the zener diode, which will turn the transistor, and your LED, on.

I forgot to add it in here, but you will also need a resistor on the base of the transistor, between the zener and the transistor.

As for the debate over whether or not a particular kit was useful... the OP did say in his post that it might turn on a relay, so linking to a prebuilt kit which accomplished that isn't all that bad. However, it's cheaper, and funner to do it on your own, and it's really in keeping with the DIY attitude of the entire modding community. Perhaps I digress, but I have a similar attitude about tea: I can buy a cup of it brewed hot from any number of locations, and it will probably taste fine, but it's pretty easy to brew it on my own, that way it's cheaper and I have more control over the final outcome.

"I've been working with electronics for about 20 years so don't listen to me go ahead and get your 555 IC solder it up then when it doesn't work spend another $20 for a premade circuit." If you've been in electronics for 20 years, then you should know that a simple timer, with a 555 or a simple RC one like I describe is not hard to build, even for a beginner.
 
I think a 555 is much easier than stuffing together 10+ cap's and resistors.. which will cost more in the long run.

Print the documentation, get a breadboard, and start connecting :)
 
It's not all that hard to give up your 555 for a genuine RC timer :)

There are 4 resistors, 1 cap, 1 zener, and a transistor. If you buy in any reasonable quantity from a site like digikey or jameco, then it'll cost you about $0.04, $0.10, $0.05 and $0.05 respectively, for a total of about 24 cents. 555 timers start at about that, and you'll still need 2-3 resistors and a capacitor, and an optional output buffering transistor.

Yeah it's a bit easier to use a 555, but it only takes a bit of effort to do this circuit.
 
mattg2k4: I've got a curiousity question about your circuit. I'll admit it's been a while since I've done any analysis on RC-type circuits and transistors and stuff, but...

I think the LED should be at the top of the transistor. Otherwise, you'll have an awfully large drop to actually forward bias the base-emitter junction on the transistors. Like, your capacitor would have to be charged to Vf of the LED + Vbe of the transistor + Vbreakdown of the zener, and I've got a feeling that that sum would be either close to, or greater than +5V, depending on the zener.

Just a hunch though, I could entirely be out in left field.
 
Ski Me said:
Hey screwbrain total cost for unit and shipping is $21.95...
Click to expand...

No, 21.95GBP = 41.74USD. That's way more expensive than any of the other solutions.
 
Sounds like your going to have to use a 555 time, a simple counter circut and a flipflop (to keep the LED on)
 
fat-tony: I'm not exactly sure, I kind of pulled that circuit out of nowhere since I know I've seen similar RC timer circuits but couldn't find any schematics handy. But it seems like your way would indeed make more sense. Or you could use a mosfet instead of an NPN, and then just make sure you were above the mosfet's gate threshold voltage and put the LED on either side.
 
lostspyder said:
Sounds like your going to have to use a 555 time, a simple counter circut and a flipflop (to keep the LED on)
Click to expand...
maybe if you wanted millisecond accuracy, but for keeping an LED on, RC will do... hes not going to the moon guys
 
fat-tony said:
No, 21.95GBP = 41.74USD. That's way more expensive than any of the other solutions.
Click to expand...
The last time I bought one from this site it was just under $22 I guess the exchange rate has gone up due to the high cost of gas. Man I remember when the dollar ruled the world now you can't even give them away overseas.
 
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