Hello All!

So I got it in my head to build this timer to control a small pump (120vac, 20w max). I could buy one for between $80 and $170usd, but don't want to spend that much and thought the project would be fun. Well i sorta built what i needed with an RC circuit and a lm555 timer ic connected to a relay. It definitely works, but the intervals are WAY too short. So i need some ideas on how to increase the cycle frequency upto about a maximum of 12 hours. (Well past the leakage threshold of my RC timer)

I don't have a PIC programer so I'd like to stay away from mcu's. I would like to use two pot's to control the off and on intervals. Control voltage can be anything between 3.3 and 18vdc.

My basic idealized circuit would be voltage regulator>timer>npn switching transister>relay. Obviously there are resisters, diodes, caps, and pot's to be added as well, but that is the basic idea i have.

Thanks for any help anyone can offer.

BTW:
I found two ic's that do what i want but they seem VERY expensive. Still maybe their datasheets will help clearify what i am asking for help building.

ICS Inc's kd6 series: http://www.ics-timers.com/pdfile/Repeat.pdf
and
Amperlite's adjustable HDFA: http://www.amperite.com/assets/Documents/hdfa.PDF

Example of commercially available timer: Green Air Products 4P Cyclestat timer

 

The ordinary 555 can't do the job and the CMOS version 7555 can do but only for a limited time.. long delays require too precise capacitor+resistor combo and the leakage from the capacitor prevents 12hrs delays..
But you can do it easily from other cheap IC's like CD4017, CD4020 and CD4060B

http://uk.geocities.com/ronj_1217/c4060s.html

 

A long range timer with some standard IC's:http://www.zen22142.zen.co.uk/Circuits/Timing/ivt.htm

Ken

 

jj_alukkas,

Thanks for the suggestions! I will research these two options straight away.

After just glancing at them I don't see a way to make two diferent timing intervals (a short on time and long off time), but i have not read the links completely yet.

Thanks again,

Michael

 

Ken,

Thanks allot! This looks like exactly what I have been looking for.

Michael

 

Well, i may have been too eager to see what i wanted to see. The circuit KMoffett suggested will provide the timing I need, but it requires that the range be selected by jumpering the the #6 pin on the 4001 to one of many pins on the 4060. This will not allow for the full range of the off, or delay, cycle to be controlled by the rheostat. I could use a dip switch to change the range from the outside of the project box, but would truly rather not.

Any ideas on how to dynamically control which range is used, and the relative delay within that range, with the pot? (I need to be able to change the length of the on and off cycles as easily as possible.)

Thanks in advance for any thoughts.

 

So using the timer at http://uk.geocities.com/ronj_1217/al1/rt.html I created this version that i think should do what i want. However, my skills are poor at best so I was hoping to get some feed back before i built it. Here is what i am thinking.

When rheostat R1 is at 0ohms the 555 cmos timer should oscillate at about 27-28Hz which divided by 2^13 (or 8192) should give an interval of about 5min. When R1 is near 750Kohms, the 555 cmos timer should oscillate at about 0.18-0.20Hz which divided by 8192 should give an interval of about 12 hours. That is the Off range i wanted.

rheostat R4 should give a range of 0-30min. This is the On range i want.

I will isolate the relay away from the board to lower the chance of interference. and use a +5v VR to smooth the input from the dismantled wall-wart transformer that will be providing the Vdd. I am sure that i have overlooked something so please feel free to point stuff out. Thanks

 

Isolation Of Relay is not necessary.. Its a small coil and wont cause any noticeable interference.. but Input Voltage must be regulated and its mandatory or else you will lose your timings along with a High value capacitor at the output of power supply will help balance sudden voltage drops.. It would be better if you use a battery instead of the capacitor..

 

I will isolate the relay away from the board to lower the chance of interference. and use a +5v VR to smooth the input from the dismantled wall-wart transformer that will be providing the Vdd. I am sure that i have overlooked something so please feel free to point stuff out. Thanks

A 78L05 or 7805 Regulator with 10uF input cap, and 0.1uF output cap, along with 0.1uF bypass caps on the ICs should be enough for stability, if the input is 9V, full wave rectified then through a 220uF cap. The current draw on the timer part is very low, so a stable voltage shouldn't be a problem. For the very long delay times, use NPO ceramics and tantalum electroylitics with the tightest tolerance you can afford. Aluminum electrolytics vary as much as -50% to + 100%, with high ESR.

I would only physically isolate the load by > 1/2" blank area on the PCB or whatever construction is built, essentially length of relay. No feedback from the line should affect the relay coil when using a proper relay.