3
\$\begingroup\$

I bought some of these LM3914 boards from eBay:

Finished board

The ad said "buyer needs to assemble it" which I was fine with. When I opened it up I saw a board like this:

Empty board

I have the LM3914 chip (DIP 18) and the LED strip. Now I'm puzzled as to how to solder them both on. It is clear from the photo above that the chip is inserted from below and soldered on top, and the LED is inserted from above and soldered underneath.

Inserting the chip I see this:

Board with chip

If I solder the chip on first, then the body of the chip covers one row of holes for the LED. And if I solder the LED on first I won't be able to solder the chip on, as they overlap like this:

Chip and LED overlap

So is there some trick to soldering this together? Or is it impossible without a reflow oven?

\$\endgroup\$
  • 4
    \$\begingroup\$ Nasty. You are saved because the board has plated-through holes, which means you can solder the hidden row later, from the side where you pushed the chip through. It heat-stresses the chip a little more, however. The left-hand variable resistor requires similar treatment (under the switch). \$\endgroup\$ – glen_geek Jan 16 '17 at 23:59
  • 1
    \$\begingroup\$ A hot air gun (reflow soldering) is helpful for these types of boards. \$\endgroup\$ – Jack Creasey Jan 17 '17 at 0:12
  • 3
    \$\begingroup\$ I think the gist of all of these answers is that you have to favor whichever of the two parts (LED array or LM3914) is most susceptible to heat damage. Without datasheets, my guess would be the LED array's filler compound is most likely to melt first. But you'll have to use your best judgement. TI's LM3914 Abs Max soldering information (PDIP) says 260degC max for 10sec max, so take a short break after every 10 seconds of soldering, to let the IC's internal leadframe cool. \$\endgroup\$ – MarkU Jan 17 '17 at 2:19
  • 3
    \$\begingroup\$ What is the problem? You just solder it from the top, using fine cone tip. \$\endgroup\$ – Ale..chenski Jan 17 '17 at 3:47
  • 1
    \$\begingroup\$ A useful trick is to put some suitable sized spacers (e.g. a couple of nylon washers) between the 3914 and the board, so it is easy to keep the chip level with more of the pins exposed on the chip side of the board where they will be soldered. From the pictures, it looks like the LED array has molded "feet" at each corner which are meant to be in contact with the board. \$\endgroup\$ – alephzero Jan 17 '17 at 6:46
13
\$\begingroup\$

Thanks to the helpful answers and comments I managed to assemble it. To help others who may have bought the same board I'll post my steps here.


Kit components

Kit components


1. Solder on 30k SMD resistors

SMD resistors

It helps to hold them in place at one end with something like PVC tape or they jump around like crazy.

The left-hand resistor is the dot/bar selector. You can omit it if you want bar mode. If you solder it you get dot mode.


2. Solder on the LED array.

Solder LED array

Note that the "+" side (anode) goes to the edge of the board (in my case the supplier had helpfully marked it with a red pen). The "-" side (cathode) is in the middle (between the legs of the LM3914 chip).

Now trim the legs so that you can fit the LM3914 chip on top of where you just soldered.


3. Solder the LM3914 chip - outside legs.

Note: - see below, you may want to solder on a 10k resistor between pin 1 and V+ to stop the low bar from lighting up faintly in dot mode.

LM3914 chip

You can insert the LM3914, turn the board over and solder one line of legs.


4. Solder the LM3914 chip - inside legs.

LM3914 inside legs

Now you can turn the board over again and solder the remaining legs from the component side.


5. Solder the trimmer pots

Trimmer pots

The two trimmer pots can be soldered. Follow the board markings. One is 5k (labelled 502) and one is 50k (labelled 503). I found it easier to solder from the component side as the legs did not protrude very far through.

Note that there are two pads on this side for which no components were supplied. The left one looks like it is for a capacitor (2.2 µF from the datasheet). The other one looks like if you jumper it then it effectively closes the switch (so you could omit the switch if you want it always on).


6. Solder the switch.

Switch

Now you can add the switch. I found it easier to solder from above (component side) particularly as I had put the trimmer pot on the other side.


Trim for desired voltage range

The switch supplied was a momentary contact switch. Presumably you are intended to press it if you want to know the current voltage. Naturally you need to press it while making the adjustments.

Connect to a voltage source, set to the high range (eg. 12V). Adjust the 50k pot (labelled 503) until the bar is at full-range. Turn counter-clockwise if it reads too low, turn clockwise if it is too high.

Now adjust the voltage source to low range (eg. 10V). Adjust the 5k pot (labelled 502) until the bar is at low-range (one bar only). Turn clockwise if it is too low (eg. not lit) and counter-clockwise if it is too high.

Repeat a couple of times as required to get both low and high ranges correct.


Note regarding low bar lighting up faintly

In "dot" mode the low bar lights faintly due to 100 µA being output on pin 1 for reasons explained in the datasheet in case you want to cascade multiple devices. This can be annoying, and to stop it you can solder a 10k resistor across that LED, as suggested in the datasheet. One way of achieving this is to solder a SMD resistor as shown by the yellow line:

SMD resistor

Of course, you have to do this before soldering in the LM3914 chip. This is a 10k resistor, not a jumper lead.


Assembly tips

  • MarkU suggests taking a break every 10 seconds when soldering the IC in order to let the internals cool down.

  • alphazero suggests using nylon spacers to help keep the chip level when soldering it (admittedly I had problems doing that).

  • Akko suggests using chip sockets (single-row ones). This would reduce the heat stress on the chip, or alternatively let you swap in different colour LEDs as required. Example sockets:

Single-row chip sockets


Theory of operation

This is a schematic of the board:

LED array schematic

The chip outputs a 1.25V reference voltage between REFOUT and REFADJ (pins 7 and 8). REFOUT is connected to RHI. Internally there are 10 comparators connected in sequence with roughly 1k resistors between each one thus they progressively compare from 1.25V to 0V between REFHI and REFLO.

For "all bars on" we would therefore need 1.25V at SIGIN (signal in) which is achieved using the potentiometer VR1 and resistor R1 acting as a voltage divider. By adjusting VR1 appropriately we get 1.25V on SIGIN when the upper voltage is present at the board input (V+). For precise adjustments you could put a meter between the middle leg of VR1 and Gnd and adjust until it reads 1.25V exactly.

For the "one bar on" reading we need to present the lower voltage range at RLO (reference low). This is done by adjusting VR2. You can work out precisely what voltage should be required here by calculating:

Vlower / Vupper * 1.25

For example for a range of 10V to 12V

10 / 12 * 1.25 = 1.042V

LED current

VR2 has a secondary purpose. The current through the LEDs is adjusted in accordance with the formula:

I = 12.5 / 5000
I = 2.5 mA

Thus the LEDs will be driven by 2.5 mA each.

\$\endgroup\$
5
\$\begingroup\$

I would solder the LED strip on first, then insert the IC. You can solder one side of the IC from the LED side of the board, and the other side of the IC from the IC side of the board. Soldering the second side of the IC may be a little harder than normal, but shouldn't be impossible.

The holes in the board should be plated, so the necessary connections will be made, regarless of which side you solder.

\$\endgroup\$
2
\$\begingroup\$

For future reference (since you already assembled the circuit) you could also have used a socket for the IC. If you use two single-row sockets, they should not block access to the LED pins, and should you accidentally damage the IC it is also easier to replace.

Alternatively, you could also use the socket for the LED instead. This would have the advantage of enabling you to get bar graph LEDs in different colors and just putting the color you want on the driver board.

Obviously, you'd have to buy sockets first if they aren't included in the set, but if you intend to build more circuits in the future, they'll most likely come in handy anyway.

\$\endgroup\$
1
\$\begingroup\$

Insert the LED array first and solder it from the opposite side. Since you have a DIP chip (LM3914) you can solder the chip pins directly into the vias from the chip side. Do this AFTER you solder the LED bar graph. It's a little more awkward than soldering from the opposite side but shouldn't be too bad.

\$\endgroup\$
0
\$\begingroup\$

I would solder the IC legs between the led display first.Then I would solder on the LED display fully and at the same time bend the chip legs back on the LM3914 IC to expose and to enable soldering of the led display legs that will later be covered by the LM3914 IC. Then solder the remaining LM3914 IC legs (i.e. bend them back into position).

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.