LED Battery Voltmeter Circuit Schematic Diagram
Simple Electronic Applications December 11th, 2009This is a low power voltmeter circuit that can be used with alternative energy systems that run on 12 and 24 volt batteries. The voltmeter is an expanded scale type that indicates small voltage steps over the 10 to 16 volt range for 12 volt batteries and over the 22 to 32 volt range for 24 volt batteries. Power consumption can be as low as 14mw when operated from 12V and 160mw when operated from 24V.
It is possible to set the meter to read equal steps across a variety of upper and lower voltages. The meter saves power by operating in a low duty-cycle blinking mode where the LED indicators are only on and consuming power briefly during a repeating 2 second cycle. The circuit may be switched to a high power mode where the active LED stays on at all times.
The heart of the circuit is the LM3914N dot-bar volt meter IC, U2. This chip is operated in the expanded-scale mode so that the circuit responds in the 10-16V range.
U2 outputs a steady voltage on pin 7 from the internal voltage reference. This is fed via voltage dividers VR2 and R5 to the internal reference input pins to set the range that the meter is sensitive to. The measured voltage is fed in on pin 5 via the voltage divider consisting of R4 and VR1. This divider scales the input voltage down to a range that is useful to the IC.
The basic expanded 12 volt scale LM3914 voltmeter circuit was published in Nuts & Volts magazine (1), a similar circuit was shown in Home Power #10 (2).
The U2 positive supply is connected to pin 3 which is nominally 12V. The U2 negative supply is switched on momentarily via transistor Q1, this switching action is what makes the circuit efficient since U1 (ICM7555) consumes a mere 0.34 ma while U2 consumes around 18ma with one LED on. The ICM7555 timer, U1 is wired to run in an astable (free-running) mode with a narrow pulse width square wave output.
The duty-cycle of U1 is controlled by the ratio of R1 and R2. R2 may be adjusted to a smaller value if faster blinking is desired, a potentiometer may be substituted for R2 if a rate adjustment is desired. R1 may be increased if a longer on-time is desired. Changes in R1 and R2 will affect the average current that the circuit consumes. The frequency of oscillation is determined by C1, R1, and R2. C1 may be either an electrolytic or poly capacitor, if an electrolytic part is used, be sure to connect the positive terminal to U1 pins 6 and 2 and the negative terminal to ground.
The output of the timer IC is fed through current limiting resistor R3 to transistor Q1 which controls power to U2. Capacitor C2 filters the control voltage input to U1 and capacitor C3 provides DC filtering for the whole circuit. When the lock-on switch across capacitor C1 is closed, the output of the timer remains on, thus enabling the U2 circuitry and increasing the current drain to 18ma. The reason the switch is not simply wired across the transistor is to keep the negative supply to U2 the same as when the circuit is pulsed on. This maintains the same calibration on the LEDs in both modes because the transistor’s voltage drop is always part of the circuit.
Last, but not least, fuse F1 protects against the potential for fire hazard should the circuit become shorted out. The average current is calculated by adding the constant current required by U1 with the product of the current from U2 times the duty cycle, see the specifications for details. To operate the circuit in the 12V mode, wire the circuit so that jumpers J2 and J5 are shorted, parts U3, C4, R6, and R7 may be left out.
When wired for 24 Volt operation, the meter responds in the 20-32V range. R6 is connected to the 24V supply instead of R4, the greater value of R6 scales the higher input voltage to a range that is useful for U2. Voltage regulator U3 with series resistor R7 scales the 24V down to a regulated 12V to provide the proper operating voltage for the ICs. Resistor R7 assures that the input voltage to the regulator stays well below the 35V absolute maximum specification of the IC. Operation in 24V mode is less efficient than in 12V mode because of the extra power dissipated by the voltage regulator and R7. To operate the circuit in the 24V mode, wire the circuit so that jumpers J1, J3, and J4 are shorted. R4 may be left out in the 24V mode.
parts :
U1: ICM7555 CMOS timer IC (Harris/Intersil)
U2: LM3914N LED voltmeter (National Semiconductor)
U3: 7812 12 Volt regulator (National Semiconductor)
Q1: 2N3904 NPN silicon transistor
D1: 1N4148 silicon switching diode
LED1-LED10: Red, yellow, amber, green, and blue LEDs in any
arrangement, see text.
C1: 1.0uF capacitor, electrolytic may be used.
C2: 0.001uF ceramic disk capacitor
C3: 10uF electrolytic capacitor
C4: 0.1uF ceramic disk capacitor
R1: 47K 1/4W resistor
R2: 2M 1/4W resistor
R3: 22K 1/4W resistor
R4: 4.7K 1/4W resistor
R5: 1.2K 1/4W resistor
R6: 15K 1/4W resistor
R7: 330 ohm 1/2W resistor
VR1, VR3: 5K trimmer potentiometer, 10 turn style
VR2: 200 ohm trimmer potentiometer, 10 turn style
F1: 1/2 Amp DC fast blow fuse
S1: miniature toggle or pushbutton switch
source : www.solorb.com
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