# Need help with a circuit and diagram for low voltage alarm, using a bicolor led flashing.

#### born2dive00

Joined Oct 24, 2016
285
You will have to forgive me, for asking but the last circuit I built was with vacuum tubes, and the computer was not yet invented. My great nephew asked me for help building his RC FPV station. I have flown rc for many years but things have changed greatly in the electronics world. What I need is a schematic to follow to build this simple circuit.

Items on hand: One 8mm Led bi color red green, one 555 timer, 2n3904-g05 transistor, 10v zanier diode, 103 10nf capacitor, 22uf capacitor, 560ohm resistor, 10k resistor, 1k resistor, n40 diode, 56k resistor, 340 resistor pizo.

Desired effect:
1. When the voltage is 12.60 – 11v green light side of the led turns on
2. When voltage is 10.99 – 10v the green side of the led turns of and the red light is flashing on once every 15 seconds
3. When voltage is 9.99v-9.5v the light flashes one second on one second off
4. When voltage is 9.49- 6v goes constant red
5. When the voltage is 9.99v the alarm chirps once every 15 seconds
6. 9.5v- 9.25 the is alarm that sounds on every 2 seconds
7. When the voltage is 9.24v-6v the alarm sounds constant
Please let me know how to do this. I can follow Schematics, but I do not know what all the individual components do. Please send a diagram with all the details I need, if I have missed something please help me out.

I thank you very much.

#### AnalogKid

Joined Aug 1, 2013
9,351
Your requirements cook down to 4 different decision voltage levels, or trip points. Traditionally, this would take 4 comparators. You also need two oscillators, or one oscillator that can be selected between two frequencies, plus something to drive the buzzer, plus some logic gating to create the various output combinations.

None of this is difficult, but there will be at least two chips, a voltage reference, and a dozen or more passives. Since someone will mention it eventually, all of this can be done with one small microcontroller if you can handle the coding.

The four trip points are 11.0 V, 10.0 V, 9.50 V, and 9.25 V. A quad comparator such as an LM339 (or quad opamp such as an LM324 operating as a comparator), a voltage reference such as an LM4040, plus a few 1% resistors. The 339 has an open collector output, while the 324 has a push-pull output. Depending on the combinatorial logic to create the output states, one might be a little better suited than the other; won't know until we get down into the details.

Read up on voltage comparators and the LM339 datasheet (it is full of circuit ideas), and we'll go for round two. Also, what will be powering the circuit? If it is the battery being monitored, that will affect some of the circuit design.

ak

#### born2dive00

Joined Oct 24, 2016
285
Your requirements cook down to 4 different decision voltage levels, or trip points. Traditionally, this would take 4 comparators. You also need two oscillators, or one oscillator that can be selected between two frequencies, plus something to drive the buzzer, plus some logic gating to create the various output combinations.

None of this is difficult, but there will be at least two chips, a voltage reference, and a dozen or more passives. Since someone will mention it eventually, all of this can be done with one small microcontroller if you can handle the coding.

The four trip points are 11.0 V, 10.0 V, 9.50 V, and 9.25 V. A quad comparator such as an LM339 (or quad opamp such as an LM324 operating as a comparator), a voltage reference such as an LM4040, plus a few 1% resistors. The 339 has an open collector output, while the 324 has a push-pull output. Depending on the combinatorial logic to create the output states, one might be a little better suited than the other; won't know until we get down into the details.

Read up on voltage comparators and the LM339 datasheet (it is full of circuit ideas), and we'll go for round two. Also, what will be powering the circuit? If it is the battery being monitored, that will affect some of the circuit design.

ak
Hey Analog kid, there will be 12vdc coming from lipo batteries.

#### eetech00

Joined Jun 8, 2013
2,565
Questions..

Is the Bi-color LED 2, 3, or 4 pin?
The fully charged 12v lipo battery output is actually 12.69v, right?

Might be able to use an LM3914. It has the comparators and reference built in..

#### born2dive00

Joined Oct 24, 2016
285
Questions..

Is the Bi-color LED 2, 3, or 4 pin?
The fully charged 12v lipo battery output is actually 12.69v, right?

Might be able to use an LM3914. It has the comparators and reference built in..
A friend of mine sent me this

#### Dodgydave

Joined Jun 22, 2012
10,050
Best way for this is a Lm3914 chip, and then feed the outputs to the cd4093 chips for flashing and buzzing.