Introduction: Schmitt Trigger VU Meter
Most VU Meter designs use ether a cascade of op amps or an LM3289 LED Bar graph driver.
For a low part count simple VU meter Schmitt Triggers are the way to go. For n LED’s it only uses
1 capacitor optional
1 diode optional
and of course n LED's
Op amps work great but you can only drive four LED’s per 14 pin DIP.
LED drivers cost $6 or more per chip.
The purpose of using a Hex Schmitt Trigger is that it is easy to find and drives six LED’s per 14 pin DIP,
this keeps board space down.
The SN74HC14N Hex Inverting Schmitt Trigger chips I used only cost $0.63 per chip.
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Step 1: Building the Circuit
Construction is quite easy. The LED’s are “backwards” because the triggers are inverting.
Also I used round LED's (I have not bought the rectangular ones yet, I know they look better
green yellow red of course)
The resistance values are not crucial as long as the one to ground is close to the sum of the others.
(I used two 100k&’s in parallel to get 50k).
You can adjust the ground resistor to get less or move sensitivity. It has A diode detector to smooth blinking of the LED’s from fast paced music, and it is optional. A larger caparitor slows down the response time so greater than 4.7uF is not good to use. (If you cand find one small enough shunt the cap with a few k& or a varible and adjust to you taste.
Step 2: How It Works (A New Kind of Meter)
If you look carefully at the schematic you will notice the input is feed into the voltage divider.
This is because the Schmitt Triggers all turn on at the same voltage (approximately 2.1V).
As the Diagrams show the divided input turns on only the ones above 2.1V.
The first two diagrams show how a 3 anf 5 volt signal is passed though the divider.
The only down side to this is that the signal must be at least 2.1V to turn the firs LED on. A pre amp
or perhaps a voltage multiplier would correct this problem. Since a BJT needs about .6V to turn on this method
will work with BJT’s (probably better with a FET since it is voltage not current controlled) as shown in the last diagram
but, transistors are not the best choice. The fact that transistors are active means it will turn on move with more
base current (voltage for FET). That causes the LED’s at the bottom to get brighter as ones further along turn
on dimly. It does not look bad on 8 or more LED displays) While a source resistor on every transistor solves the problem
it adds to the part count and that’s the opposite of what I am trying to do. (Although it still has less parts than older
transistor VU meter designs) They are still a good option if you do not want to use DIP’s.
Step 3: More Uses
Of course it can be used as a Volume Unit meter but,
it will (like all VU meter) work as a battery tester. It though
will not need the zener diode because the Schmitt triggers will
fire at a 1.8V every time.
Also like any Bar graph
and countless others