Digital timer - Page 1
Forum section: electronics / general

author: Lotus
author_rank: Contributor
timestamp: February 19, 2013, 11:40:19 am
content: I need a digital timer which can wait something like 5 minutes (adjustable time), then turn on a relay for 1 minute, and i would like the timer to do this forever.I dont think oinary digital timers can do that, does any of you know how to do that?

author: sacherjj
author_rank: Frequent Contributor
timestamp: February 19, 2013, 11:50:08 am
content: Do you ever need to change the timer or will it always be 5 mins/1 min?How accurate do you need it to be? Is 4:45 or 5:15 OK? Is 50 secs or 70 secs OK?The quickest thing I can think of to put together is an ATTiny45 Micro. You can download add ons to program it in Arduino. In your loop function, have delay statements to make the 5 mins. Then throw a pin low and delay 1 minute. Then throw that pin high and repeat.The low pin goes to an N Channel MOSFET with a clap diode powering the relay. Add filter cap and the micro and you have 4 components.You can program the ATTiny45/85 with normal ICSP or use an Arduino board as an ICSP if you don't have one.

author: Guppzor
author_rank: Contributor
timestamp: February 19, 2013, 12:10:56 pm
content: Wouldn't a 555 timer setup be simpler than a micro?

author: Lotus
author_rank: Contributor
timestamp: February 19, 2013, 12:24:37 pm
content: The accuracy is not important, as the timer will have to ventilate the air in a heated chamber, so 1 minute +- 10 seconds wouldnt really matter, and also drift in time wouldnt matter either.What is important is that the circuit forever switch on/off a perhaps SSD relay at more or less the time i specify

author: Gall
author_rank: Frequent Contributor
timestamp: February 19, 2013, 12:57:11 pm
content: Quote from: sacherjj on February 19, 2013, 11:50:08 amOr even ATTiny4, ATTiny5, ATtiny9, ATtiny10 which is much smaller (just 6-pin SOT-23).QuoteIf simplicity is number of required parts, then no. A micro does not require any external components to work, a 555 does.

author: fcb
author_rank: Super Contributor
timestamp: February 19, 2013, 01:23:01 pm
content: I wouldn't bother with a microcontroller, some other simple ways:1. Use a 74LS123/4098B (dual retrig monostables), set one half at 1 minute and the other at 5mins. No programming...Look at Fig 15:http://www.rapidonline.com/pdf/83-0426.pdf2. Perhaps if I wanted more stability (using smaller low tempco capacitors) I might use an astable configured 555 timer, a divider (such as 4020B/4040B) and a 4017B (configured as a 1:6 divider).3. If you have a handy secondary of a transformer around, then tap off (via a 1M resistor and a zener) the 50Hz, feed it into a chain of 4017B's set as simple dividers (10, 10, 10, 3) then feed the exact once per minute pulse into another 4017B (configured as a 1:6 divider).

author: SeanB
author_rank: Super Contributor
timestamp: February 19, 2013, 04:02:10 pm
content: Google asymmetric recycler, if you want a ready built unit. Analogue or digital ones are quite cheap.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 19, 2013, 04:20:02 pm
content: A small (8 pin) PIC would be your best bet.

author: N2IXK
author_rank: Frequent Contributor
timestamp: February 19, 2013, 06:03:54 pm
content: You can buy time delay relays with 2 adjustable delay intervals specifically to handle repeat cycle or interval timing applications like this.The last time I needed one, I went with the TRDU series from SSAC:http://www.ssac.com/files/downloads/30/TRDU.pdfTiming intervals adjustable by a pair of DIP switches, from fractions of a second to over 1000 hours. If you only need 1, and it has to be reliable, buying a commercially made, prepackaged solution like these is generally a better idea than designing and building your own.