Question Battery Life Indicator

GC64

.
Joined
Jul 3, 2016
Messages
177
Likes
151
Location
Georgia
Portables
2
I am trying to build a battery life indicator:

I have a couple of questions about it:

1. Will it be fine if I simply hook up my 7.4v batteries to the circuit?

2. About what voltage is a 7.4v battery about to die, so that I can set it at that?

3. Which model of transistors I should use? It doesn't specify..

Thank you
 

cheese

the tallest memer in town
Staff member
.
.
.
.
Joined
Mar 2, 2016
Messages
2,671
Likes
2,333
Location
Florida
It'll be fine if you hook it to your 7.4v, they usually run around 3.2v-3.4v for an almost dead cell afaik, so 6.4v-6.8v. Assuming you've got protection circuitry, you can run your batteries dead and time it, then run them most of the way and measure the voltage. Finally, these are npn transistors (they're specified by the direction of the arrow, easy way to remember, npn=Not Pointing iN) , I assume any old npn will do fine.
 

GC64

.
Joined
Jul 3, 2016
Messages
177
Likes
151
Location
Georgia
Portables
2
It'll be fine if you hook it to your 7.4v, they usually run around 3.2v-3.4v for an almost dead cell afaik, so 6.4v-6.8v. Assuming you've got protection circuitry, you can run your batteries dead and time it, then run them most of the way and measure the voltage. Finally, these are npn transistors (they're specified by the direction of the arrow, easy way to remember, npn=Not Pointing iN) , I assume any old npn will do fine.
Thank you. I was in fact aware that I had to use npn transistors for the circuit, but I was uncertain if a specific model mattered.. Does it matter at all? I have a few different kinds so.. Also one more more noob question: I would use the potentiometer to adjust which voltage I want the led to turn on, right? Here's another image of the circuit that contains the potentiometer:
 
Last edited:

Aurelio

The Fixer ᕕ( ᐛ )ᕗ
Staff member
.
.
.
.
.
.
Joined
Mar 3, 2016
Messages
1,864
Likes
2,527
Portables
2
This circuit doesn't need neither fast nor high current transistors, so basically you should be fine with whatever you have with you, just make sure that the one that you have fits with the circuit voltages. Anyway daftmike used some BC547 if you want to compare the specs.
 

GC64

.
Joined
Jul 3, 2016
Messages
177
Likes
151
Location
Georgia
Portables
2
I have two that are fairly close to BC547's specs:

2N3904 / C945 / BC547
Collector − Emitter Voltage: 40 / 50 / 45
Collector − Base Voltage: 60 / 60 / 50
Emitter − Base Voltage: 6 / 5.0 / 6

2N3904 is probably closer to it than C945, plus most of it's voltages are lower rather than higher, but which is the best option? 2N3904 or C945?
 

Gman

RTFDS
Staff member
.
.
.
.
.
Joined
Feb 25, 2016
Messages
1,011
Likes
2,200
I think either one you use, the circuit will still function as desired. I've built the low battery circuit using BC547s and 2N3904s and they both work the same. The circuit uses a trimmer potentiometer so it can be adjusted.
 
Top