Morning all

I am working on something a little unusual. I am using a number of LED's to show the amplitude of a signal.

Let's say 5 LEDs which each switch on when V>=x where x is different for each LED. Therefore as V rises each LED will switch on in turn and off as V falls.

Simple so far. However V is not only the variable to be measured but also the only voltage to the circuit. Therefore there is no stable power supply or voltage reference that could be used to compare V to.

The idea I am running with is to use an NPN in common collector configuration with a potential divider biasing Vbe so that each transitior switches on at a different level of V. A second NPN is used to current limit the stage so as V rises the current to the LED remains <~20mA.

I must admit this is my first foray into the practical applications of transistors. My spice simulations look promising but it is evident that, although the transistors turn on at different voltage levels the LED bank will look like it turns on as a group with LEDs individually getting brighter. I would like a more digital output. Any ideas for a second stage (current comparison maybe?)

The circuit below is repeated for each LED, with R46 controlling the voltage across Vbe.





Thanks for looking.

 

You could use a regulator to bring down the input voltage (to be measured and to be used as supply) to a low voltage regulated voltage. Use this as the supply voltage for your circuit. For the measurement inputs, divide the input voltage with resistor dividers. It could be cryptic, but I hope you get the idea!
BTW, it is nothing unusual, as bar graph led driver ICs do this.

 

You could use a regulator to bring down the input voltage (to be measured and to be used as supply) to a low voltage regulated voltage. Use this as the supply voltage for your circuit. For the measurement inputs, divide the input voltage with resistor dividers. It could be cryptic, but I hope you get the idea!
BTW, it is nothing unusual, as bar graph led driver ICs do this.

RamaD. Thanks for your reply.

The voltage to be measured ranges between 0 & 20V. The measured voltage and the pre-regulated voltage (to use your example) would be from the same source.

I hadn't seen bar graph driver IC's before; thanks for the heads-up. I have looked at a datasheet and again these are not suitable for my application as the signal source and the IC power source are the same voltage. For example when my signal source is in the 0 to 3V the IC is not powered ideally for my application i would like to have an LED lit on multiples of 2V.

 

You can do something similar with only LEDs and resistors.

http://postimg.org/image/afz7ytest/

 

Do it right and use one of these:

or better, note the price

 

Your requirement to operate from a 0V supply is … challenging.

 

Nah, piece of cake. His time-averaged voltage is not 0V, it's somewhere between 0 and 20V. Just use a peak detector (diode plus capacitor) to capture enough energy to run an LM3914 long enough to coast through the down times.

Now, the source has to have enough current to not be significantly effected by the the power to the chip and the LEDs. The OP hasn't told us about the source impedance.

 

Nah, piece of cake. His time-averaged voltage is not 0V, it's somewhere between 0 and 20V. Just use a peak detector (diode plus capacitor) to capture enough energy to run an LM3914 long enough to coast through the down times.

Now, the source has to have enough current to not be significantly effected by the the power to the chip and the LEDs. The OP hasn't told us about the source impedance.

As clever as this energy harvesting method is, we have no assurance that there will be sufficient power. The signal could well spend long periods of time at the low end of the range.

 

Agreed. But since the OP is hoping to light LEDs, the power needs of the LM3914 would be relatively trivial. But who knows. More details needed.