# Need help building a circuit for low voltage alarm with bi color LED with flasher

Jan 26, 2017
60
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.

#### Alec_t

Jul 7, 2015
3,457
Welcome to EP.
While it would be theoretically possible to do what you want using discrete components, the sheer number of voltage thresholds and timed periods involved would require a bucket-load of parts. Have you considered using a microcontroller instead?

Jan 26, 2017
60
Welcome to EP.
While it would be theoretically possible to do what you want using discrete components, the sheer number of voltage thresholds and timed periods involved would require a bucket-load of parts. Have you considered using a microcontroller instead?
Hello Alec, Im lucky I can navigate the internet at my age, LOL. What if we made it more simple,

1. When the voltage is 12.60 – 11v green light side of the led turns on
2. When voltage is 10v the green side of the led turns off and the red light is flashing on once every seconds
3. 9.5v- 9.25 the is alarm that sounds on every 2 seconds

#### Alec_t

Jul 7, 2015
3,457
That would be more easily doable. I question the chosen voltage thresholds, though. Assumig you are monitoring a lead acid battery, it's my understanding that battery damage can occur if the voltage drops below 10.8V?

Jan 26, 2017
60
That would be more easily doable. I question the chosen voltage thresholds, though. Assumig you are monitoring a lead acid battery, it's my understanding that battery damage can occur if the voltage drops below 10.8V?

I am monitoring a 3s lipo battery, across all cells. a fully discharged lipo battery is 9v, 3v per cell

#### Alec_t

Jul 7, 2015
3,457
For lipo you need to monitor individual cells, to prevent any one discharging too far. Do the cells have individual protection circuits already?

#### Audioguru

Sep 24, 2016
3,650
RC Li-Po batteries have three identical cells bonded together. The individual cells discharge voltages are not monitored, only the total discharging voltage. The cells are balanced charged where each individual cell is monitored.

I think 3V per cell is too low and damage is being caused. 3.2V would be safer.

#### Pyramid

Jan 17, 2017
24
Maybe something like this:

R11 and R12 scale the voltages by 2.8x so you can power the opamp by 5VDC (same with the 4093 logic). The opamp will need to be a rail-to-rail input type. The LED should be a 3 lead device for simplicity. The buzzer should be a self exciting piezo buzzer for simplicity. You may have to play with the time constants of R14/C1 and R16/C2 to get the flash/beep rate you like. Voltages are approximate but close to what you spec'd.

Jan 26, 2017
60
Maybe something like this:
View attachment 31580
R11 and R12 scale the voltages by 2.8x so you can power the opamp by 5VDC (same with the 4093 logic). The opamp will need to be a rail-to-rail input type. The LED should be a 3 lead device for simplicity. The buzzer should be a self exciting piezo buzzer for simplicity. You may have to play with the time constants of R14/C1 and R16/C2 to get the flash/beep rate you like. Voltages are approximate but close to what you spec'd.
Pyramid, Thank you, One quick question for you, What is blue triangle/chevron on J1, what is the VCC, and I am assuming by ground you mean connect to the negative terminal of the battery, and not ground as in a ground to earth, I mean physically grounding the circuit to the green grass with a copper wire correct?

#### Pyramid

Jan 17, 2017
24
J1 is the input from the battery stack. I assume you wanted to power the circuit from the batteries? The ground connection would also be to the battery minus.

Vcc is the supply to the opamp (pin 4) and the 4093 chips (pin 14) which both run off 5VDC.

Jan 26, 2017
60
J1 is the input from the battery stack. I assume you wanted to power the circuit from the batteries? The ground connection would also be to the battery minus.

Vcc is the supply to the opamp (pin 4) and the 4093 chips (pin 14) which both run off 5VDC.

Hey Pyramid
Ok got it, what is U1A, U1B, U1C do you have a part number or identification of these 3 items? what is an opamp?

Last edited:

Jan 26, 2017
60
RC Li-Po batteries have three identical cells bonded together. The individual cells discharge voltages are not monitored, only the total discharging voltage. The cells are balanced charged where each individual cell is monitored.

I think 3V per cell is too low and damage is being caused. 3.2V would be safer.
Hey audioguru, dead lipos are at 3v/cell X 3 cells=9v, damage occurs at 2.7-2.8v/ cell X3 = 8.1v This is where damage is going to occur, we regularly discharge our lipos down to 3.0v/c and the have never shown any damage, i.e. loosing charge/ capacity.

#### Pyramid

Jan 17, 2017
24
Hey Pyramid
Ok got it, what is U1A, U1B, U1C do you have a part number or identification of these 3 items? what is an opamp?

U1A/B/C is an op amp with rail to rail inputs (the input voltage range includes the supply voltage and ground). You will need at least the positive rail input i.e. a TL064 type op amp.

Op amps are high gain, high input impedance, low output impedance, differential amplifiers that can be configured for different functions. As configured in the schematic, they are used as voltage comparators. When the voltage is greater on the (+) input than the (-) input, the output is high. When the (-) input is higher than the (+) input, the output is low.

The input voltage is divided by 2.8 (by R11 & R12) so that the thresholds of the opamps are less than 5VDC (the supply for the opamp). If you multiply the threshold voltages by 2.8x, you will have the voltages of the battery stack that trigger the desired outputs.

Jan 26, 2017
60
U1A/B/C is an op amp with rail to rail inputs (the input voltage range includes the supply voltage and ground). You will need at least the positive rail input i.e. a TL064 type op amp.

Op amps are high gain, high input impedance, low output impedance, differential amplifiers that can be configured for different functions. As configured in the schematic, they are used as voltage comparators. When the voltage is greater on the (+) input than the (-) input, the output is high. When the (-) input is higher than the (+) input, the output is low.

The input voltage is divided by 2.8 (by R11 & R12) so that the thresholds of the opamps are less than 5VDC (the supply for the opamp). If you multiply the threshold voltages by 2.8x, you will have the voltages of the battery stack that trigger the desired outputs.

Thank you pyramid we will try this once we get back from vacation, Im taking my nephew to a large scale rc plane show over this next week.

#### Pyramid

Jan 17, 2017
24
I just realized you may be able to simplify the circuit by gating the flasher and beeper oscillators directly:

The 4093 has 4 gates in one package, so you would only need 1 chip.

Jan 26, 2017
60
Maybe something like this:
View attachment 31580
R11 and R12 scale the voltages by 2.8x so you can power the opamp by 5VDC (same with the 4093 logic). The opamp will need to be a rail-to-rail input type. The LED should be a 3 lead device for simplicity. The buzzer should be a self exciting piezo buzzer for simplicity. You may have to play with the time constants of R14/C1 and R16/C2 to get the flash/beep rate you like. Voltages are approximate but close to what you spec'd.

Pyramid, I just want to check with you on the 2 schematics they are measuring the total voltage correct? i.e. 1 line, 12v-6 volts correct? What I mean is if I have a lead acid battery with a total charge of 13v, the circuit will work correct? if I connect a 3s lipo battery with a 12.6v charge the system will work, if I connect a transformer ac to dc 120vac to 12vdc the circuit will work. What I am trying to find out is does the schematic monitor individual cells like using lipo, or does it monitor the total charge on the line?

#### Audioguru

Sep 24, 2016
3,650
With a 5V supply, a CD4093 can drive the LED with about 3.5mA (dim) without a current-limiting resistor or with about 2mA (very dim) with a 330 ohm resistor. For 5V supplies I use a 74HC132 quad Schmitt Trigger Nand that can supply up to 25mA (very bright) if the current is limited. I do not know if the buzzer will work with the very low current from a CD4093 but it will work fine from a 74HC132.

#### Pyramid

Jan 17, 2017
24
Pyramid, I just want to check with you on the 2 schematics they are measuring the total voltage correct? i.e. 1 line, 12v-6 volts correct? What I mean is if I have a lead acid battery with a total charge of 13v, the circuit will work correct? if I connect a 3s lipo battery with a 12.6v charge the system will work, if I connect a transformer ac to dc 120vac to 12vdc the circuit will work. What I am trying to find out is does the schematic monitor individual cells like using lipo, or does it monitor the total charge on the line?

The circuit does not monitor individual cells. It measures the total input voltage, but the thesholds are fixed by the resistors between 5V and ground.

The opamps work as voltage comparators. They are comparing the (divided down x2.8) input voltage from the battery to a fixed reference voltage that is derived from a stable 5V source, so the voltage references will not change, only the input voltage from the battery. Therefore the thresholds where the opamps change state will also be fixed (11V / 10V / 9.5V**) regardless of the fully charged voltage of different batteries. If the battery stack was 48VDC, the thesholds would still be 11-10-9.5V. If you need different thresholds for different battery types, you will need to adjust the resistors between ground and the 5V regulator. Setting the resistor values is basic Ohm's law: The current through the string of resistors is 100uA and the total resistance of all resistors must equal 50K. To get the first (lowest) voltage: R1=V1/1E-4. The next higher voltages are the difference between the current and next lower voltage: R2=(V2-V1)/1E-4. R3=(V3-V2)/1E-4 etc. The final (top) resistor will be: R=(5V-V3)/1E-4.

**If you divide these voltages by 2.8 you will get the voltage thresholds on the schematic. The input voltage from the battery is divided by 2.8 via R11 and R12 because the input to the opamps must be between 0 and 5VDC.

Jan 26, 2017
60
The circuit does not monitor individual cells. It measures the total input voltage, but the thesholds are fixed by the resistors between 5V and ground.

The opamps work as voltage comparators. They are comparing the (divided down x2.8) input voltage from the battery to a fixed reference voltage that is derived from a stable 5V source, so the voltage references will not change, only the input voltage from the battery. Therefore the thresholds where the opamps change state will also be fixed (11V / 10V / 9.5V**) regardless of the fully charged voltage of different batteries. If the battery stack was 48VDC, the thesholds would still be 11-10-9.5V. If you need different thresholds for different battery types, you will need to adjust the resistors between ground and the 5V regulator. Setting the resistor values is basic Ohm's law: The current through the string of resistors is 100uA and the total resistance of all resistors must equal 50K. To get the first (lowest) voltage: R1=V1/1E-4. The next higher voltages are the difference between the current and next lower voltage: R2=(V2-V1)/1E-4. R3=(V3-V2)/1E-4 etc. The final (top) resistor will be: R=(5V-V3)/1E-4.

**If you divide these voltages by 2.8 you will get the voltage thresholds on the schematic. The input voltage from the battery is divided by 2.8 via R11 and R12 because the input to the opamps must be between 0 and 5VDC.
With a 5V supply, a CD4093 can drive the LED with about 3.5mA (dim) without a current-limiting resistor or with about 2mA (very dim) with a 330 ohm resistor. For 5V supplies I use a 74HC132 quad Schmitt Trigger Nand that can supply up to 25mA (very bright) if the current is limited. I do not know if the buzzer will work with the very low current from a CD4093 but it will work fine from a 74HC132.

Thank you Pyramid, got it, the power supply will be between 13.2- 12v if a battery it will drop down to say 9.5-9.24v, if it is a ac to dc power supply it should never drop below (in theory) 12v.
I chose those values because they are pretty universal. between battery and power supply types.
Just to be clear with this circuit are we talking about the values in post 1 or 3?

Audio guru suggested using a 74HC132 quad Schmitt Trigger Nand ??? what is this, is it needed, and where does it go?

This the LED I am using,
https://www.alibaba.com/product-detail/-LED-Special-DIP-LED-3MM_60448391347.html

What voltage Pizo would I need?

Replies
3
Views
2K
Replies
1
Views
655
Replies
3
Views
910
Replies
20
Views
4K
Replies
4
Views
714