Available on Tindie is a new kit by Kuzyatech- an LM3916 based VU meter/bargraph driver.
This kit builds on a famous LM3916 VU bargraph driver chip from TI/National semiconductor. The chip is essentially a constant current 10 segment driver fed by an array of comparators. It also provides internal reference voltage to set the rail of the precision resistor network feeding the comparator reference inputs. A signal coming in then determines how many signals are lit. Depending on the mode selected, either bargraph or a single dot is displayed. This particular kit is using LM3916, optimized for audio meter applications but could also be used with LM3914 and LM3915 parts to get linear and log response respectively (mainly set by the individual resistance values of internal divider networks).
Design features and notes:
- R6 is a placeholder should you require a voltage divider or a filter on the input
- R3 is normally a jumper, but could be used to offset the trigger points up if needed (If building a battery monitor for example, where you need more attention in a narrow voltage range)
- When using with audio input, set R2 to 0 ohms. That effectively moves voltage reference adjust pin to ground, setting top comparator rail to 1.25V
- R5 is only needed when connecting multiple units in a chain. Use P2 and P1 to get at the signals needed for that
- Jumper P5 enables bargraph mode when installed.
- The board will run happily at 5V or even lower. If you change LED current setting and run from higher supply voltage, keep an eye on power dissipation in the driver. (See page 2 of the datasheet for further details)
- Per customer request, here are the input voltage range and power dissipation figures:
The abs max supply is 25V for the chip, but you have to keep in mind power dissipation inside which is a function of LED drive current and number of LEDs on at any moment. See for example http://www.ti.com/lit/
ds/symlink/lm3916.pdf page 11. These chips have junction to ambient resistance of 55C/W and max junction temp of 100C, so you’d take max operating ambient temp, and back out from that how much power can be dissipated. Say you run from 12V and use Red leds with 1.8V forward voltage. The board is set to around 5-6mA per LED. The power dissipation in the chip is then: (15-1.8)V*0.06A or 0.792W. At 55C/W this will raise the chips temp 43C above ambient. At 12V, you’ll only see 33C rise. If you use blue LEDs (2.9V Vf), you’d see 40C and 30C respectively.
Measured values: with 15V in, LM3916 had case temp of 62C when running red LEDs and 56C with Blue. Ambient was 18C, so the estimates were pretty close.
The better way:
The most controlled and convenient test signal source is a good signal generator. I’ve been using a slope mode on my Rigold DG1022, with very slow (300-500mHz) frequency and and varying symmetry to obtain the effects in the test video
The other way:
In the absence of a signal gen, an audio output from a computer can be used. Just make sure to set potentiometer R2 to 0 ohm to get the full range.
Assembly instructions in pictures:
There are several unusual steps in these instructions, so please make sure you read them before building the kit!