For proper operation of this wireless thermometer, reference current (to pin 4 of DAC0808 – IC22) of the receiver unit should be pre-adjusted. To do this, follow the steps below:
Connect a known voltage source (not exceeding +5V) to any input of the ADC, say, at IN6 (pin 4) of the ADC. Switch on the transmitter unit. Connect a DMM across R26 of the receiver unit. Set the range switch to DC 200mV range, positive lead to ground and negative lead to top of R26. Switch on the receiver unit. LEDs at decoder outputs should start glowing to indicate the received voltage data. If source voltage is 1.5V, status of LEDs should be as listed in Table I.
So, received voltage = (D× 5)/256 = (76×5)/256 = 1.50
where D is the weight of the binary numbers represented by LED7 through LED14.
Now, adjust trim potmeter VR1 to get 150.00mV on the dial of the multimeter. Connect another voltage source at the input and see that the multimeter shows it correctly. If required, re-adjust the trim potmeter. After proper calibration, enclose the circuit in two separate boxes with suitable connections of input and LED indicators.