The Duke chapter of EWH constructed a breadboard prototype of the timer in the spring of '08 using an RC circuit (see schematic image). The heart of the device is the capacitor which can be used as a timer by letting it slowly discharge. Initially, the capacitor is charged to a certain voltage which is determined by the adjustable voltage divider. Flipping a switch starts timing by allowing the capacitor to start discharging. The voltage across the capacitor is compared with a reference voltage from a second voltage divider with a thermistor in it. The thermistor changes the reference depending on the temperature. When the capacitor voltage drops below the reference, a buzzer or an LED is activated to indicate that the proper time has been reached. However, the amount of time it takes will depend on the reference voltage, and ultimately on the temperature. Using the correct component values leads to a time vs. temperature function which is very close to the one desired.
I've uploaded a matlab code in the attachments section which simulates the device's output.
Oct 4, 2008
Last week, we finished initial tests at various temperatures. We found that the capacitor, which has a +/- 20% tolerance, seemed to have a value 10% lower than its label. We weren't able to check it directly, but the shape of our time-temperature plot indicated a value around 300 uF. To compensate, a 33 uF capacitor was placed in parallel, and this improved the performance. Slight adjustments are still needed for the potentiometer, but they will depend on the actual values of the components in each device we build.
Oct. 19, 2008
I've uploaded a price list. The device will cost around $14 for an open circuit board. Adding an enclosure with battery pack will add around $10-$12, and adding an external power supply will add around $12 as well. We have also found that the thermistor we were using is now obsolete, so instead we will now use a 220 K thermistor and replace the 10 K resistor with a 220 K resistor. This will lower the current draw during timing. The 3.3 M resistor will be replaced by a 2.7 M resistor in series with a 1 M potentiometer, so that we can adjust for real capacitor values. A three position switch will replace the 2 position switch so that the device can also be turned off. The three switch positions will be off, reset, and time. Finally, a buzzer will replace the LED.