Voltage reached LED indicator circuit request

Thread Starter


Joined Nov 26, 2008

I am using the TPS61200 low voltage synchronous boost converter to provide 4.1 volts from a .5 volt solar cell source. The device works great, but I would like to incorporate a simple low current draw LED indicator to let me know when the 4.00 volts is reached. So that if I do not have a meter, I would know the light source was significant enough to engage the device. Typically the converter needs greater than .5V to get started and then the output voltage jumps to 4.1 volts. I would like to incorporate either a momentary LED or one that would require that would only light if the voltage is at 4.00.

So basically - does anyone have a simple low current draw circuit that would light an LED only when the input voltage is greater than 4.00 volts?
I would also like to use all SMT parts to save space.

I apologize in advance if this is an easy question as this is only a hobby for me. I tried searching for this concept, but did not find an exact match on the forums.




Joined May 7, 2007
Two LEDs (or combination of diodes and an LED) in series with a resistor and a momentary switch would be the simplest I think.


Joined Nov 29, 2005
As the circuit that would turn on the led would be preferred to be supplied with the same rising ~4V supply, a comparator chip capable of running with less than 4V (3.3V) with a voltage reference IC could turn on the led.

Less precise could be a 3.9V or 4.3V zener that would conduct when such voltage is reached to turn on a led in series to it. It would not be an abrupt on/off; would have some gradual glow in the vicinity of the sensing treshold.


Joined Nov 20, 2005
lets get simple just series resistors and diodes to drop the voltage until 4v.


Joined Dec 20, 2007
A 0.5V solar cell is probably very small with a low output current. Its power might not be enough after boosting the voltage to be enough to light an LED bright enough to be seen in the daytime.