LED displays of all sizes and brightness are available , a best buy is to get a three to four digit stick that is already multiplexed . For those who are not familiar with their use I include some details and drawings for your information . Use lengths of about 3" of rainbow coloured #24 wires for easy identification as a harness and solder each end to the circuit board . Make sure you identify correctly which wire goes where .
As a reference you may wish to open up a new window to view the circuit while on this page .
The circuit was designed to use Common Cathode LED displays which I already had in my junk box . Any size and colour can be used as long as you can get the right colour filter for their use.
Important Note : For common cathode Pins 6 of the14543 IC must be connected to the negative bus to pins 7and 8 .
Using common anode display
The design can easily be modified for the use of Common Anode LED displays as follows ; Refer to the circuit "Counter Display"
1 - Still using PNP transistors , reverse their polarity so that the collector is now connected to digits 1 to 3 , the base connections remain the same as connected to the 1K resistors and the emitter will now be connected to the positive bus by doing the changes described in para 2.
2 - Connect pin 6 of the 14543 IC and the transistors emitter to the positive supply bus instead of pin 7 and 8 as shown .
3 -The 14543 is now connected for Common Anode operation .
Details of the Rotary switch wiring is shown below . As an option two more ranges will be added when a SPST switch is used to parallel a 1.uF capacitor to the .1uF of range 3 &4 . If you install the rotary switch on the foil side don't forget to reverse the order of the resistances and capacitors on the switch . Also shown is the dimension of the switch to fit the PCB .
Circuit diagrams for the Low Voltage Monitor and the Voltage Regulator are shown. A suitable voltage regulator of your choice can be used , a TO220 1.5 A regulator must be used as the current will reach up to 130 mA in certain conditions , no heat sink is required .
The Low Voltage Battery Monitor circuit shown is calibrated for a 9 volts supply , as it reaches a low voltage of 7 volts the LED will come on indicating need for batteries change .
R1 and R2 form the calibrated set point voltage divider to pin # 3 of the 741 and can be replaced by a 10K variable resistance to calibrate for other voltages above 6 volts.
Introduction construction-1 Construction-2 Construction-3 Calibration