Car battery voltage monitor: The monitor uses a two-digit 7- ‘ segment LED display to indicate the battery voltage. In addition to this, there are ‘low’ and ‘high’ voltage warning indicators. The display range is 9-18 V.The battery voltage is scaled down by a factor 7.83 by resistors R20 and R17. This means that the voltage range at the SIG input of the LM3914 (IC3) is 1.14-2.29 V for battery voltages between 9 V and 18 V. The LM3914 has an internal voltage reference that maintains a voltage difference of 1.25 V between pins 7 and 8. Each of the ten resistors in the ladder network thus drops a voltage of 0.125 V. The voltage difference for each step between 9 V and 18 V is (2.29-1.14)/9, or 0.1278 V. which is close enough to 0.125 V for the resolution required by the present application. Here, the LM3914 is operated in ‘dot’ mode.The open-collector outputs of the LM3914, 1.1\-L10\, are connected to 1 1,(12 pull-up resistors, nine of which are trained in SIL array R19. The resistors also cause a ‘1’ to appear on the most significant (MS) display for all input voltages ( greater than 9 V. Only if 9 V is measured will the base of T1 remain high, so that the MS display is quenched. If the input voltage is higher, the base of T1 is pulled low, and the B and C segments of LDS light, showing a ‘1. Output L2 of the LM3914 is not connected because the priority encoder, IC), automatically encodes a ‘0’ if none of its inputs is pulled low. Together with the ‘1’ on the MS display, this produces the reading ’10’. Inverters IC2a-IC2d ensure the correct input levels for the inputs of the 4511 display driver, IC1.Inverters IC2e and IC2f function as undervoltage (<8.7 V) and overvoltage (>18.7 V) monitors and indicator drivers.
The actual switching thresholds will depend on the IC used, so some experimentation with the value of R15 (overvoltage) and R14 (undervoltage) may be called for. The undervoltage LED should light at input voltages between 5 V and about 8.5 V. Lower voltages are not possible because the LED light would not be seen anymore. The overvoltage LED should light at an input voltage of about 19 ‘.The circuit is simple to adjust with an accurate digital voltmeter and an adjustable power supply. If these instruments are not available, simply set preset P1 to mid travel, which is a sufficiently accurate setting in most – cases. If precision is needed, apply 12.0 V to the input of the circuit. Adjust P1 until the display reads ’12’. This value should remain on the display if the input voltage is varied between 11.5 V and 12.5 V. If not, adjust P1. Next, reduce the input voltage to 8.0 V. The undervoltage LED should light, and the display read ‘0’. Slowly increase the voltage to 9 V. At a value of 8.5 V the display should change to ‘9’. The exact voltage at which this happens is not terribly important. Next, apply 17 V, which should cause the display to indicate 417′. Slowly increase to 18 V, and check that the display changes to ’18’ between 17.2 V and 17.8 V. If this does not work, R21 is out of tolerance, which causes an incorrect step size of the ladder network. The cure is simple: replace R21 (and, if necessary: R17 and R20 also) by a 1% tolerance type.The circuit draws a fairly high current, which causes voltage regulator IC5 to run hot if it is not fitted with a small heatsink.The printed circuit board consists of two sections (display and voltage monitor proper), which must be separated by cutting before they are populated. The connection between the two units so obtained is formed by a 12-way pin header, K1, on the main board, and a mating 12-way SIL socket, K1′, on the display board. The display unit is mounted at a suitable position on the car’s dashboard, with the displays covered by a bezel. Alternatively, the driver circuit and the readout may be fitted into a small plastic enclosure, for which a suggested front panel layout is shown.
Parts list
Resistors:
R1-R4, R6, R7, R9, R10, R11• R16 = = 470 Ω
R5. R.8 = 220 Ω
R12, R13 = 10 kΩ
R14 = 3.9 kΩ
R15 = 1.5 kΩ
R17 = 1.2 kΩ
R18,R21 = 1Ω
R19 = 9-way 10 k Ω SIL array
R20 = 8.2 kΩ
R22 = 560 Ω
R23 = 250 Ω preset H
Capacitors:
C1-C4, C6, C7 = 100 nF
C5 = 47 uF, 40 V
Semiconductors:
D1 = LED, yellow, 3 mm
D2 = LED, red, 3 mm
D3 = 1N4001
T1 = BC557A
LD1, LD2 = HD1 107G (Siemens)
Integrated circuits:
IC1 = 4511
IC2 = 74HC04
IC3 = LM3914
IC4 = 74HC 147
IC5 = 7805
Miscellaneous:
K1 =12-way angled SIL header
K1′ = 12-way SIL socket 1
Plastic enclosure HM-Kit (Pactec).
61x97x23mm (WxDxH));
order code 6600-902.
Printed circuit board Ref. 93401E
