Electronics > Beginners
Creating a simple device that tells you when your dishwasher has been run
jesuscf:
LM555 to the rescue! Since the LM555 has a couple of voltage comparators and a S/R flip/flop it is fairly simple to make a circuit to behave as the OP wants.
Below is the circuit diagram. It contains an LM555, a LM317 voltage regulator, and a LM335 temperature sensor. The voltage out of the temperature sensor is proportional to the temperature. To calculate the temperature in degrees C=(V(pin 2 Lm335)-2.73V)*100. The voltage regulator is set to 4.56 V using the 10k potentiometer. The LED turns on when the voltage of the LM335 reaches 4.56V*(2/3)=3.04V which corresponds to about 31 degrees C or about 88F. For a different temperature adjust the voltage of the regulator accordingly. Even if the temperature drops below 31C the LED remains on until cleared by pressing the push button attached to pin 2 of the LM555.
This picture shows the circuit cold, not triggered yet.
I used the soldering iron to trigger it in this picture (about 77C). The LED is now on:
After cooling down, the LED is still on in this picture:
After pressing the push button, the LED is off:
grifftech:
if the temp is correct just use some wax in a tube attached to a magnet, flip to reset.
TerminalJack505:
I think Ian.M has the best idea, which is to use an SCR. The OP is looking for an analog solution.
My non-analog solution would be to use a Schmitt-triggered flip-flop with asynchronous clear.
analogueAdder:
I love that 555 design. I would run it of AA batteries and use a 1K resistor for the LED to improve the battery life but those are small details.
reboots:
I endorse the LM555 approach. But since I have a schematic sitting around which demonstrates a latching relay configuration, I'll include that.
The functional block at bottom left shows a DPDT relay (a coil plus two sets of switched contacts). The disc thermostat would connect in place of the "UP" pushbutton, applying power to the relay when the thermostat closes. Once energized, the relay supplies itself with power until the connection is reset using a normally-closed pushbutton ("TOP_LIMIT"). The application in the schematic, a motorized lift, uses the second set of relay contacts to switch the load; but if the indicator light is connected in parallel with the relay coil, a single-pole relay could be used as well.
As others have observed, this approach would quickly exhaust a set of batteries. A wall transformer or other continuous supply of power would be needed.
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