Monday, June 22, 2009

More on that timer

Modifications made to the timer:
  • New "wall wart" power supply. Oscilloscope test of output shows it has no filter capacitor so a 470uF 25V electrolytic was added to the board. No-load voltage with filter cap was >20VDC.
  • Added a LM78L12ACZ and LM78L05ACZ voltage regulator
  • A 1N4002 was added as a safety feature in case power supply polarity is inadvertently reversed.
Here's the schematic:

MCU = Microchip PIC 16F785
Q1,Q2 = S9012 PNP transistor
Q3 = 2N7000 MOSFET transistor
XTAL = 32.768kHz crystal
7SEG LED = ELD-306IDB dual 7-segment LED common anode
all resistors 1/4Watt 5%

I've installed the timer in my aunt's kitchen. Much to my chagrin the volume of the buzzer was not close to earsplitting. Although as with any sonalert buzzer, it will surely be heard tens of meters away.

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