Low Power LED Voltmeter by LM3914

This 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.

Different colored LEDs may be used for the voltage level indicators, this allows the battery state to be read in the dark. With the new blue LEDs, it is possible to have a nice looking rainbow of colors using two each of red, amber, yellow, green, and blue LEDs. The circuit will also work with inexpensive and common red LEDs. If the circuit is to be used in sunlight, ultra-bright LEDs should be used, although even those may be hard to read without some kind of sun shield.

Typical uses include the monitoring of portable battery operated systems and indoor wall mounted home power system charge indicators. The cost of the parts for the circuit is around $25.00 (US) and the parts are commonly available, except for the optional blue LEDs. If blue LEDS are used, expect to pay a premium for them, they cost several dollars each, compared to around 15 cents for the other colors. The blue LEDs do look nice in any case.

The circuit may be built with either the CMOS ICM7555 timer or the more common bipolar 555 timer. The 7555 timer will provide much more efficient operation and should be used for systems with small batteries. The volt meter works nicely with the solar charge controller and low voltage disconnect circuits described in the home-brew section of Home Power #60 and #63.

Logic Probe w/Pulse Injector

This is an useful and easy-to-build Logic Probe with Pulse injector. A schematic of a “digital version” is included.

Here is the schematic of my Logic Probe with Pulse Inyection:

Logic Probe with pulse inyection

If you work with digital circuits, TTL or CMOS, you may know sometimes is not easy to diagnose a problem or know the reason of an unexpected behavior in the circuit. That is the function of the Logic probe, It helps to monitor the signal on every connection. As the pulse injection is included, it also can help to send some pulses to the circuit. This is a perfect tool when developing circuits.

I built this one on 1989 when I was at the school learning about digital electronics. I saw one that does the same thing, monitors and injects pulses, with a cost of a few hundred dollars. I built mine for only $7 USD. I don’t have any picture because that was more than 15 years ago, but I remember clearly that I used a toothbrush case. My friends were astonished how perfectly it worked.

How this works:

The power for the logic probe is taken from the circuit to monitor. As a CMOS is used, it works with 3V to 15 Volts. The tip is connected to the point in the circuit that needs to be monitored. To inject pulses, just close the switch. Pulse injection is used when working with latches, flip-flops, etc.

Here is another version of the same circuit that shows when the signal is “1″, “0″ or “P”:

Schematic Digital Logic Probe

I don’t use a logic probe anymore because I have enough computers to use simulators, however, soon I will start working with digital circuits and I will build my own Logic probe with pulse injector. By the way, this logic probe does NOT recognize the third state or hi-z.