Andrew M. Bishop
Description
This project uses a Microchip PIC microcontroller, a serial EEPROM and a thermistor to create a temperature recorder.
Performance
The temperature is measured and stored at user programmable intervals; this can be from 1 second to 256 seconds. The time interval is set by programming it and the start time into the EEPROM.
Most of the time the PIC will be asleep and the EEPROM IC is inactive. This gives a very low current consumption of approximately 50 uA or about 1 mAh per day.
The EEPROM used is 32kBytes which can store up to 32,000 measurements. This could be one measurement every 30 seconds for 11 days for example.
The combination of thermistor and analogue circuit gives a range of between about -40 °C and +100 °C although the linear range is between about –10 °C and +40 °C.
Pictures
Complete Unit
In this picture the temperature recorder can be seen with the external thermistor plugged in. The connector on the end contains power, I2C clock and data and analogue input.
The overall size of the complete unit is 40 mm long, 26 mm wide and 16 mm tall.
In this picture the temperature recorder is connected to a battery pack with 4xAAA batteries. This gives a good indication of the size and shows that I need a much smaller set of batteries.
PCB (version 1)
The first version of the circuit is built on stripboard with DIL packaged ICs and conventional through-hole passive components.
The PIC microcontroller is to the left of centre on the PCB with the EEPROM IC mounted vertically to save space on the right of centre.
The external connectors are the 1.3mm power socket on the bottom left and the 5 pin SIL socket on the right.
The other components are the ICSP socket with resistor and diode at the top left, crystal oscillator and two capacitors on the left of the PIC, the two I2C pull-up resistors next to the EEPROM and the resistor for the thermistor potential divider at the top right.
PCB (version 2)
The second version of the circuit is built on a homemade single sided PCB with SO8 packaged ICs and surface mount passive components.
The ICs have a pitch of only 0.05" (1.27 mm), the decoupling capacitor is an 0805 package (0.08" by 0.05" = 2 mm x 1.27 mm) while the resistors are in a 1206 package (3 mm x 1.5 mm). The crystal and its capacitors are the same as before as are the connectors. The tracks are all routed with 0.02" width (0.5 mm) although in the final production they have come out slightly wider.
To be continued
