3
\$\begingroup\$

Some background info first:
In my country, price of electricity depends among other things on the time of the day when energy is used. Because of that, many people tend to turn on big energy consumers after midnight, when electricity is cheaper. One of my friends needs a new laundry washing machine, so I went browsing with her. We noticed that many new machines have a delayed start option which would be very useful because it would be possible to fill the machine and program it and it would start working by itself during cheaper time. I don't think that it would be too complicated to make a circuit which will start a washing machine after X amount of time.

More relevant part:
After examining my washing machine, I noticed that machine itself is started by a normally off pushbutton. I'm thinking that the button could be replaced by a transistor or some sort of normally off relay. Anyway that's not (yet) the problematic part for me.

The question itself:
How would I design a circuit which would wait for several hours and then "push" a button without using microcontrollers? I know that a microcontroller is the most elegant solution, but I'd rather skip collecting necessary gear for working with them at this time. Discrete logic gates, flip-flops or anything else which doesn't need a programmer is acceptable for me.

My first idea is to create or find prefabricated some sort of counter which would be connected to a slow oscillator. The oscillator would have very low frequency (say 2Hz if possible) and the counter would send signals to a device which would respond when counter reaches a certain number (say after 18 000 seconds).

I was thinking about using 74LS162/3 counters. In 74LS162's datasheet under minimum frequency it says 25MHz if I'm reading it right, so I guess that it doesn't fit my needs, but 74LS163 has 0 as minimal frequency so it looks like a logical choice to me.

As for the oscillator, the slowest crystal I could find is 32 768 KHz which is too fast for me. I thought about using capacitor and coil to make my own oscillator. Some (maybe too) basic calculations tell me that L*C needs to be 1.989*10^-2. What capacitor/coil combination would be good for that?

I didn't think too much about power supply, but I'd either use "wall-wart" adapter or check the insides of the washing machine for any available power sources.

The last part of my question are vibrations. Are there any special considerations for circuits which are going to vibrate? I was thinking of using one of those solderable protoboards for basic circuit board.

I know that a book can be written for every point, but at this moment I'm just looking for some rough ideas.

\$\endgroup\$
  • 8
    \$\begingroup\$ If I were you I would consider repurposing a kitchen timer or an alarm clock. \$\endgroup\$ – joeforker Oct 8 '10 at 21:34
  • 1
    \$\begingroup\$ @joeforker Why should something be simple when it can be complicated:). Actually I never would have thought of finding a useful purpose for my stack of alarm clocks which I never used. While it definitely wouldn't be as educational as making a circuit from scratch, it would quickly solve my problem. I'll take it into consideration. \$\endgroup\$ – AndrejaKo Oct 8 '10 at 21:40
  • \$\begingroup\$ Use the open switch / closed switch / beeping to power a 555 circuit that will close a relay for just long enough to start the washing machine. Have it send you an SMS? No need to undercomplicate things just because you are using an off-the-shelf long delay timer. \$\endgroup\$ – joeforker Oct 8 '10 at 22:37
  • \$\begingroup\$ There are lots of timers that are used to turn lights on and off -- they have a switched 110v outlet (in the US). You could use one of these, set it for the start time, and use the output to control a 110v relay which would operate the washer button. Set the off time on the timer as close to the start time to release the relay. (If you have to press and release the pushbutton to start the washer, you may need a little more complicated relay arrangement.) \$\endgroup\$ – tcrosley Oct 9 '10 at 3:22
  • \$\begingroup\$ An unexciting practical answer: tinyurl.com/3t2htbr - a mechanical timer wall switch. About $12. Mount it straight into the control panel plate of the washing machine... \$\endgroup\$ – Toybuilder May 24 '11 at 20:32
4
\$\begingroup\$

CMOS 555 timers can be used to achieve delays of hours. A C of 4,700µF and R of 15 Mohm would time for 7550 seconds, or 2.09 hours (2 hours, 5 minutes, 50 seconds.)

Due to the input current on bipolar 555 timers, they cannot be used with such large resistors, similar to how you cannot use high valued resistors with bipolar op-amps.

\$\endgroup\$
4
\$\begingroup\$

I know you specifically said that you did not want to use a microcontroller, but I think you should consider taking the plunge into the microcontroller world. Doing a timer like this is very easy to do on a very cheap micro since you don't need it to do anything special.

In general I have found that using a "complex" circuit of discrete components is usually plagued with minor errors in both design and construction that cause them to take a lot of time and money.

\$\endgroup\$
  • 1
    \$\begingroup\$ Absolutely. For US$20 you can get the parts together to make some very accurate timing systems (e.g. Arduino bootloader, DS1307 RTC, etc) \$\endgroup\$ – user1307 Oct 9 '10 at 5:14
  • 2
    \$\begingroup\$ In general I agree that microcontroller would be the best solution and plan on taking that route one day, but in my specific case that would require either buying one of the kits available in my country (which are 100€-150€ depending on accessories) or convincing one of my relatives to borrow me credit card (since I as a student don't have a steady source of income and can't get a credit card myself) and then pay more for shipping than for the components being shipped. Still, you get +1 for good suggestion. \$\endgroup\$ – AndrejaKo Oct 9 '10 at 8:45
4
\$\begingroup\$

Have a look at basing your circuit around a CMOS 4060 14-stage ripple counter and oscillator. I have used these in the past for long interval timer projects - before PICs came along :-)

-=mike=-

\$\endgroup\$
  • 1
    \$\begingroup\$ I'll take a look. It looks promising to me. \$\endgroup\$ – AndrejaKo Oct 10 '10 at 10:37
4
\$\begingroup\$

I think the best solution is to use CD4060 with a crystal oscillator which can produced 1Hz pulses. Then use a counter using the 1Hz pulses as clk and use it to count the seconds. There are problems with 555 timer approach. The resistors and capacitors use to set the frequency are not all that accurate, and they drift with temperature. Actually the 555 Chip itself can drift with temperature. Check this site: hackersbench.com

\$\endgroup\$
3
\$\begingroup\$

For what its worth...

I implemented a circuit on my dryer for programmed start with an Omron PLC. There is a button for ARM and a button 4hr / 8hr setting. The PLC has 6 inputs and 4 outputs. The outputs go to the buttons and one is paralleled with the dryer start switch as you mentioned. (might be a relay in there too for the dryer switch)

The PLC was a cheap one I bought second hand on ebay (Omron CPM1A). IT takes 120v AC as an input and provides a 24v DC output for driving relays (or switch lights in this case).

Works well. It was interesting to program with ladder logic too.

\$\endgroup\$
  • 1
    \$\begingroup\$ Talk about overkill. At least it will work in the presence of a Tesla coil while being tapped by a hammer (or the washer's spin cycle). \$\endgroup\$ – joeforker Oct 12 '10 at 20:13
  • \$\begingroup\$ Yah, I know, it's kinda funny. I had the PLC around from another project. Works great though. \$\endgroup\$ – JeffV Oct 12 '10 at 22:39
2
\$\begingroup\$

buy one of these that fits your countries power system, they make digital ones too that aren't much more expensive if you need greater control.

Plug your devices and an AC relay rated for your countries power into it. Use the relay's output to "push your button" however that needs to be done for the particular device.

\$\endgroup\$
  • \$\begingroup\$ The simplest and most pre-fabricated solution.. is ususally the winner. \$\endgroup\$ – Anonymous Type Jan 10 '11 at 0:16

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.