| Electronics > Beginners |
| Coffee Pot Shut Off Timer Project |
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| Old Printer:
My wife recently bought a perc style coffee pot that works really well, but it has no long term timer to shut if off if we forget. Something in the 4-6-8 hour period would probably do nicely. That time frame seems a bit long for an RC circuit. I have thought about having an arduino handle the counting and I have a relay that will handle 12 A @ 125V and it can be triggered with a current of 5-24V at only 1.5mA. Dose anyone have a suggestion on the control circuit aside from what I have already mentioned. I do have some pro mini arduino's in my junk box as well. Thanks, Dave |
| Paul Price:
You could do this with a single CD4060 counter chip. It has a built in oscillator and can count the oscillator output to a max of 2 to the 14th power. When the chip is reset and starts counting, 2 to 13th x period of osc seconds later the output could trigger your shutoff relay. You can add a pot to control the oscillation freq if you want to go fancy and have a knob to set the time. 2 to the 13 is 8192, so if the oscillator is set to .1Hz, then it would take 8192 x 10 secs oscillator output or 81920 seconds. That's quite a few hours, do the math. You can set a higher oscillator freq or use a different output pin to shorten the time. The CD4060 doesn't need a regulated voltage but you shouldn't exceed 19 volts or there can be smoke. One chip, a diode, two resistors and a transistor might be needed to activate the relay. Two resistors and a small capacitor will set the oscillator working. Add another resistor and a reset switch and you're in business. Now, the problem remains on how to power your timer. Will you use batteries or a wall wart? Of course any microchip/Arduino can do this, but in some ways this can be more complicated if you are new to programming and setting up the software. Also in this case you might need to use two power supplies voltages and a driver transistor for the relay. |
| Nominal Animal:
My suggestion is a DigiSpark (or a clone, or a Pro Mini if you already have those), a 10k linear pot for adjusting the time, an I2C 128x32 OLED display for showing how long (hours and minutes) since the coffee was brewed (started to brew), and optionally a couple of tactile buttons. You'll also need a 5V DC supply, or a 9-12V supply and a transistor with a current-limiting resistor for driving the relay. The microcontroller should only be powered when the coffee machine is turned on. The relay should be normally open, and control the power to the rest of the coffee pot. When turned on, the microcontroller starts counting time, and energizes the relay. (For safety, I'd actually use one tactile button, so that the relay is not energized by default, but only after the button is pressed. This does add a button press to the brewing, but it also means that if you can use a hardware watchdog timer to reset the microcontroller in case it glitches or crashes, it will not automatically energize the coil; and so the pot would default to "off" if a bug occurs.) Normally, the time in hours and minutes elapsed is shown on the display, so it shows how long since the brewing started. If the pot changes by more than some hysteresis value, or the other button is pressed, the maximum on time is displayed (and varied by the pot reading); after a couple of seconds, it'll revert to showing the elapsed time. To turn off the coffee pot, you turn off the coffee pot from its power switch. When the set time has elapsed, the relay is de-energized, and you change the display to show it is on, but the coffee pot is off. I would show how long the pot has been off, so that one can still grab a late cup if they want. I'd probably put the time on the right, as big as I can make the fixed-size font (needing only the ten digits), and an optional OFF / SET label smaller to the left of it. Without the label, the display shows the duration since the brewing started. I'd probably blink the colon between hours and minutes, too: one second on, one second off. (At 16×32 pixels, ten digits require 640 bytes of Flash; at 32×16, the OFF and SET label need 64 bytes each. Total 768 bytes. The DigiSpark ATtiny85 has 6k of flash for user code, so I think it should all fit. You can't use any of the existing SSD1306 libraries for the OLED display, as they're too large, though; you'd need to edit one heavily to get to work on an ATtiny85. No such issues with Arduino Minis, of course.) |
| Nusa:
If you just want a solution it's not expensive: https://www.amazon.com/Century-Digital-Countdown-Repeat-Function/dp/B01D3QEK4E/ https://www.amazon.com/Century-Mechanical-Countdown-Timer-Grounded/dp/B00MVDTEXS/ |
| Old Printer:
Thank you all for the replies. I will digest the info provided. The Arduino version is attractive because I think I have all the necessary components on hand. I don't have the skills to write the code off-hand, but I have done a bit and there is so much out there that could be adapted. On the other hand the CD4060 would be something new and that is always a plus. My wife would urge me to go with the "Amazon" solution as she does not even trust UL, but that's no fun :) This is a few years old hobby for me and I enjoy every opportunity to put into practice the things I have learned. Thanks! |
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