LED Bargraph battery monitor part 2 · 9 October 08
OK. So You’d like to build a set of those neat little bargraph battery monitors!
Here’s the first article: part 1.
- LM3914 bargraph display IC
- 10 DIL LED bargraph display
- 10 uF electrolytic capacitor
- 100R resistor
- 1K resisitor
- 270R resistor
- 10K resistor
- 22K resistor (for the 6V version)
- 200R precision cermet trimmer
- 500R precision cermet trimmer
- IN4001 silicon rectifier diode or similar.
- a piece of stripboard 10 holes x 16 holes (strips run along the 16 hole side)
All resistors should ideally be 1% or 2% tolerance.
The electrolytic capacitor needs to be very small and short if you want to mount all the boards together as shown below.
I’m going to write this article as if you are building the bargraph for 6V batteries. If you are using 8V batteries, change the 22K resistor for a 33K resistor, If 12V then use a 56K resistor.
Please note that all components in these diagrams are shown from the bottom, as if you were looking through the board from the copper strip side.
1. Cut the stripboard into rectangles which are 10 tracks wide and 16 “holes” long.
2. Use a small drill bit or track face cutter to break the tracks like so:
Make absolutely sure you have cut all the way through the tracks. I blew a couple of LEDs in one of the modules by missing a hair’s breadth of track joining cut sections.
Please note: If you would like bar display mode instead of dot display mode (which I don’t recommend) then omit the second drilling on the bottom line.
3. solder in the links like this:
Links can be made from tinned wire without insulation such as cut off ends from resistors and diodes etc. Make sure the ends of the links do not protrude far through the board because you don’t want them to interfere with the next board if you are mounting them together.
4. insert the chip and verify that you have it in the right place and the right way around. Remember my diagrams are shown from the bottom. Once you have done this then carefully bend the chip pins toward the center line of the chip (like a dead spider) to retain it. I used the edge of the desk but it can scratch the desk. Don’t solder them yet.
5. You have a 10 LED DIL bargraph module a bit like your chip only it has ten pins per side rather than nine. One side will be anodes, one will be cathodes. Find the side with the cathodes and lay the ten pins against the ten track ends right next to where the chip is mounted. The circuit board should be between both sets of pins on the LED module and against the cathodes. There should just about be room for the pins beside the chip legs you haven’t soldered yet. Now solder the pins in place. Nine of them connect directly to the chip pins. The last one is just above the marked end of the chip.
Good. Well Done!.
Solder the rest of the chip pins in if you have not done so already. The picture shown has everything soldered but it does show the way the LED module is mounted on the end of the board.
6. Add the resistors now. I recommend the order to be 100R,22K,10K,1K and 270R then the two variable resistors, 200R and 500R. Remember we are looking from underneath here.
Things to note:
There isn’t going to be very much room: Maybe keep the 100R resistor a couple of mm off the board to as to be able to fit the 22K resistor in because one end of the 22K plugs in directly under the 100R resistor. Keep the 22K resistor close to the board because there need s to be room for a diode later. The 1k resistor need to be at least partially standing up as it’ mounting holes are too close otherwise. same goes for the 270R resistor which need s to be completely standing up (in a “u” shape with the resistor on one side and a length of lead on the other). It’ll also need to be pushed over a bit to reduce it’s height. The 270R resistor does something a bit sneaky here because of space. One of it’s leads emerges into one of the track cut-outs you made earlier. See the diagram above.
The 200R and 500R variable resistors are straightforward to mount. Note that they are staggered with respect to one another. Please bend the leads inwards over the tracks just like with the chiip to prevent them sticking too far below the board.
7. solder in the diode with the cathode (the painted end) towards the chip. This diode will prevent current from passing to the chip’s power supply if the monitor’s terminals are reversed.
8. get the small capacitor and bend it’s negative lead (normally next to the stripe) directly outward at the case. Bend the positive lead so that it follows the first lead but a couple of millimeters lower down further away from the case. The two leads should be parallel now and pointing away from the case to one side (the side with the stripe) Now poke the leads through the board as show in the diagram. The capacitor shoulld be lying down directly over the diagonal wire link. Trim the leads and solder it in.
9. As shown on the picture below: You see the 10 anodes on the LED module which are not soldered yet? OK. Bend the first and last pins down by a few degrees, maybe 20. Now fashion a bit of wire so that it sits against all the pins, under the 8 straight ones and over the two bent end ones. Leave a couple of centimetres free at the end near the notch on the chip. This need s to go through the board next to the Vcc (+) pin on the chip which is pin 3 it is the same track as the positive lead on the capacitor. Make sure everything is straight, Solder the wire up to the board then all of the LED module anode pins. It’s a bit fiddly.
10. The boards are connected by leads or pins on the top and bottom track of the board on the other end to the LED end i.e. close to the variable resistors. The positive track is the one which is closest to the adjusting screws on the variable resistors. It also leads to the diode. The negative track is the other one.
The positive track and the adjustments mark the “top” of the board i.e. the bargraph display starts from the bottom of the board closest to the the negative supply at lower voltages and finishes at the top of the board closest to the positive supply.
One mounting idea for attaching lots of these boards together is to attach short leads, half a centimeter to the tracks pointing back away from the board (I put mine through the board first)
Then the boards can all be soldered next to each other to a master boards which is also 10 “holes’ high. This keeps the spacing correct and your LED modules can be glued to each other with superglue to make a rigid multi display. Each individual element is exactly 1 inch high, 0.4 inches wide and 1.95 inches deep.
James May in the UK