I'm not sure that this is the correct place to ask, but I have had great success with questions on the other stackexchange sites, so I'll try my luck :)

I need a device, which can press a button on a given time, which I should be able to set. It's meant for turning on my washing machine in the middle of the night. So I would want to put the device over the turn-on button before I go to bed and then have it press the button at a time, I've set it for.

If anybody knows a place to get something like that, I would love to hear about it else I would be willing to build it myself, if I know what to look for.


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    \$\begingroup\$ It's the right place if you want to make such a gadget, but not really if you just want to buy one (i.e. shopping questions are generally closed) Do you have any electronics experience? You could do this with a simple timer IC and solenoid. \$\endgroup\$ – Oli Glaser Dec 24 '12 at 0:18
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    \$\begingroup\$ I don't mind building it myself :) I don't have any electronics experience, I am however a computer scientist and have met some basic electronic logic? \$\endgroup\$ – Niels Sønderbæk Dec 24 '12 at 0:32
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    \$\begingroup\$ Plug into a wall power timer, then set the start to ON. \$\endgroup\$ – Optionparty Dec 24 '12 at 3:50
  • \$\begingroup\$ Sounds like you need one of these: homecare-huisautomatisering.nl/images/schakelklok.jpg \$\endgroup\$ – jippie Dec 24 '12 at 10:04
  • \$\begingroup\$ I wish I could just use that, but there need to be power on for the button to be pressed, so a wall power timer wouldn't be enough. \$\endgroup\$ – Niels Sønderbæk Dec 24 '12 at 13:37

You could use an Arduino microcontroller and wire it with a relay inside the button panel of the washing machine. This might be a little neater than using a solenoid and wouldn't require you to take it off when you aren't using it. You could have a dial (potentiometer) to adjust how long it will wait (1 hr, 2 hrs, 5 hrs, etc.) before momentarily clicking the relay. A little indicator led could be used to show it is counting down. An Arduino Uno might would be a bit overkill, but would allow plenty of customization (LED/LCD display, different methods of input) and also expansion (you could also control your dryer or other appliances). I would be happy to give you more information, I don't know how much experience (if any) you have in any of this.

  • \$\begingroup\$ +1: Only I would keep the relay and 'tronics outside of the machine but doing the same thing. \$\endgroup\$ – Everyone Dec 24 '12 at 12:40
  • \$\begingroup\$ I can't get inside, as that would mean I would have to break it up, so I can't get access to anything inside. It doesn't have to be neat, just have to be functional :) \$\endgroup\$ – Niels Sønderbæk Dec 24 '12 at 13:28
  • \$\begingroup\$ If you're sure you can't get inside it, then a solenoid would work fine. Do you want to be able to remove it from the machine? You wouldn't have to, you could put a button that starts it after the certain amount of time, and one that starts it right away. \$\endgroup\$ – Thanson Dec 24 '12 at 14:22

I'll assume for this answer that you are not familiar with microcontrollers (let me know if you are, makes life a bit easier)

The solenoid driving part is reasonably easy, so we can look at that first:

Places like Farnell, Mouser, Digikey, etc have plenty of choice solenoid-wise, here are a few examples. You don't say how much force is needed to push the button but I'll assume it's not too much, so we can pick this part from the list (it's 12V, but there are 6V and 24V version there too)

Push solenoid

To drive it, a simple circuit like this will do:

Solenoid Driver

The MOSFET is an N-channel type, there are many options for this part, for example an IRLML0030 will do fine here. R2 is to make sure the gate is not left floating, something like 10kΩ will do here. R1 and the LED can be left out (unless you want an indicator) and D1 can be something like a 1N4002 or similar.

For the timing there are plenty of options, one of the simplest might be to get a cheap digital alarm clock and hack into it to get at the alarm buzzer wire, then use this to drive the gate of the MOSFET - set the alarm accordingly and you're done (alarm clock ground and source of MOSFET need to be connected too) Obviously the alarm clock buzzer signal needs to be a constant DC, which is not guaranteed, but you would have to test this first.

The other option is to set up a counter IC like the CD4541BE and use the Q output to drive the gate. It's up a 16-bit counter, so if we assume you need to set it for 6 hours (e.g. start before you go to bed at 10pm and machine will activate at 4am) then you would need the oscillator to run at:

2^16 / (3600 * 6) = ~3Hz

The datasheet gives the formula for calculating the RC oscillator frequency as:

1 / (2.3 * R * C)

So if we take something like R = 145kΩ and C = 1uF, then we get:

1 / (2.3 * 145kΩ * 1uF) = ~3Hz

This will not be very accurate since it uses an RC oscillator, but I'm assuming for your application high accuracy is not too important. I picked a low value for C, since you can use a 1uF ceramic rather than an electrolytic which have horrible tolerances. For the resistor, since 145kΩ is not a standard value, you can add a couple of 1% parts to get close enough (e.g. 100kΩ and 47kΩ, which are both standard values)
The datasheet gives an example circuit, but here is another one for a "one shot" application:

CD4541 circuit

I'll add a complete circuit later if the above seems a bit random - particularly the power and solenoid timing needs mentioning (a large capacitor across the supply to provide enough initial force is an option) but this should hopefully get you started with ideas.

Okay, here is a complete circuit - forgive the messy presentation as it was a bit rushed (and has not been tested obviously)

Button Pusher

You will notice I have altered the voltage and Rtc/Ctc values on the left hand side, to provide the 3Hz needed for the 6 hour time limit (Rs can stay the same as it was).
The hand drawn black lines between the circuits are connection wires. Both points labelled +12V connect to the 12V wall wart (try and get one rated for > 100mA to make sure you have more than enough current)

The wire can be anything you have to hand really (say the thickness of doorbell/speaker wire will do fine)

The 470uF capacitor next to the solenoid will be an Aluminium electrolytic type (these caps are polarised, so make sure the plus side goes to +12V line, this is usually the side with the longer lead), the value is not too important (in fact both it and R1 are not strictly necessary, but will provide some protection in case anything shorts, and also provide an initial boost of current for the solenoid in case your supply has little current to spare)
C2 can be a ceramic/plastic film, or electrolytic, this turns the solenoid off after about half a second or so (since you don't need it to keep pressing the button when activated)

All the parts you need are shown above, and some wire, solder and soldering iron is the only other stuff you need (plus a way of mounting your solenoid over the button)

When I have more time tomorrow I will try to replace the above with a better schematic. If you have mo questions in the meantime, ask away :-)

  • \$\begingroup\$ That really helps a lot! I am confused about what you mean by 1% parts. What kinds of resistors would I need? \$\endgroup\$ – Niels Sønderbæk Dec 25 '12 at 1:15
  • \$\begingroup\$ I know it's also a lot to ask, but beside the elements you already provided me links for, could you tell me what else I actually need to get - i'm guessing wires and other stuff? \$\endgroup\$ – Niels Sønderbæk Dec 25 '12 at 1:17
  • \$\begingroup\$ Sorry, I mean 1% tolerance parts in reference to the resistors. This means they are guaranteed to be within 1% of their stated value (e.g. for a 1kOhm 1% resistor, it will be somewhere between 1010 Ohms and 990 Ohms. You will see the tolerance listed with the part specs (typical values are 5%, 1%, 0.1%, etc) \$\endgroup\$ – Oli Glaser Dec 25 '12 at 1:19
  • \$\begingroup\$ No problem, I will update the answer with some more detail on all the stuff you need as soon as I can. 2 questions - do you have any experience with microcontrollers? How much force (roughly) is necessary to push the button? \$\endgroup\$ – Oli Glaser Dec 25 '12 at 1:22
  • \$\begingroup\$ Thank you so much! Just take your time, I do realize that this is Christmas time. I can't really express it in technical terms, but not very much, the 12 V should be plenty. I haven't had any real experience with actual microcontrollers, but I just had a course in machine architecture and learned some basic theoretical microcontroller design :) \$\endgroup\$ – Niels Sønderbæk Dec 25 '12 at 1:22

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