Big Digit Driver Assembly Instructions

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This page walks through the basic assembly process for building the Big Digit Driver.

Bill of Materials, or, "What's in the kit?"

Line Ref Des Description Type Vendor Vendor Part Number Quantity per Board
1 N/A PCB Big Digit Driver board Evil Mad Science LLC N/A 1
2 R25 Current Sense Resistor for DP 22 ohm, substituting 24 ohm 1/4 watt carbon film resistor Digikey 24QBK-ND 1
3 R1 through R5 DP Load Resistors 68 ohm 1/4 watt carbon film resistors DigiKey 68QBK-ND 5
4 R11,R13,R15,R17,R19,R21,R23 Current Sense Resistors 16 ohm 1/4 watt carbon film resistors Digikey 16QBK-ND 7
5 R12,R14,R16,R18,R20,R22,R24,R26,R27 Base Resistors, Power LED Resistor 1k ohm 1/4 watt carbon film resistors Digikey 1.0KQBK-ND 9
6 D1, D2 Linear Regulator Load, DP Load (Diodes) 1N4740A, 10V, 1 watt Digikey 1N4740A-ND 2
7 U9 Shift Register Socket 16 pin 0.3" Socket Digikey A100206-ND 1
8 U10 Schmitt Trigger Socket 14 pin 0.3" Socket Digikey 3M5462-ND 1
9 JP1 thru JP5 SPI Ground Jumpers/Schmitt Trigger Bypass Zero ohm 'Resistors' Digikey 0.0QTR-ND 5
10 Q1 thru Q8 Digital Control Transistors 2N3904, NPN Transistor, 40V, TO-92 Digikey 2N3904-APCT-ND 8
11 U1 thru U8 Current Regulators LM317L, Adjustable Linear Regulator, 0.1A, TO-92 Digikey LM317LZ-ND 8
12 U11 Linear Regulator TL750L05, 5V, 150mA, TO-92 Digikey 296-8002-5-ND 1
13 C2 Bypass Capacitor 10uF Ceramic Capacitor Digikey 445-4816-ND 1
14 C1, C3, C4 Bypass Capacitors 0.1uF Ceramic Capacitor Digikey BC1148TR-ND 3
15 LED1 Power LED 3mm Yellow LED Digikey 754-1212-ND 1
16 J2, J3 Screw Terminal 2 position euro-style screw terminal DigiKey ED2635-ND 2
17 J5, J6 SPI Headers 6 pin (2x3 DIL) 0.1" Header Digikey 609-3202-ND 2
18 J4 Power Jack 2.1mm ID Barrel Jack Digikey CP-002AH-ND 1
19 U9 Shift Register 74HCT595, 8-bit 16DIP Digikey 568-1560-5-ND 1
20 U10 Schmitt Trigger 74ACT14, 16DIP Digikey 296-4368-5-ND 1
21 J1 Display Connector 11 Position/22 Contact 0.156" pitch board edge connector Digikey EBM11DSAN-ND 1
22 N/A Display Top Standoffs Standoff, 7/8" round Digikey 1848K-ND 2
23 N/A Display Bottom Standoffs Standoff, round, 3/4" DigiKey 3488K-ND 2
24 N/A Display Top Standoff Washers 1/16" thick Nylon Washers, #6 screw McMaster 90295A090 2
25 N/A Display Bottom/Driver Board Standoffs 1/8" tall Nylon standoff McMaster 94639A407 2
26 N/A Display Bottom/Driver Board/Mounting Screws Screw, Button Socket Cap 6-32 x 1/2" McMaster 92949A148 6
27 N/A Display Top Screw, Button socket Cap 6-32 x 3/8" McMaster 92949A146 2
28 N/A Delrin Insulator Delrin Insulator Evil Mad Science LLC N/A 1

Assembly procedure

This document will show you where each part goes.

This guide assumes that you have had some soldering experience. If not, or it's been a while, you may also want to look at pages 2 and 5-7 of the Bulbdial clock assembly instructions, which go over basic tools and technique. And, if you really are a beginner, there are plenty of good tutorials on line, like this one.

1. The circuit board

Here's what the bare circuit board looks like:



We'll be adding components to the labeled locations there.

When we refer to locations on the board and say things like 'look on the left-hand side of the board', we assume the board is being held with the silk-screen side up (we'll be calling this the 'front side') and with the text right side up.

2. Decimal Point Current Sense Resistor

Let's start with an easy one! Locate the 24 ohm resistor (color code: Red/Yellow/Black/Gold).


Bend the leads as shown.


This will go in location R25 - it's right above the power jack footprint on the right-hand side of the board (location J4).


Drop it in place (location R25), and bend the leads over by about forty-five degrees on the back of the board. With these resistors the orientation doesn't matter.


Solder the leads and clip off the extra wire.



This is the general procedure we'll follow for all the parts in this kit.


3. Decimal Point Load Resistors

Let's finish off the decimal point resistors.

We'll be installing 68 ohm resistors (color code: Blue/Gray/Black/Gold) in locations R1 through R5.


All of these locations are immediately to the left of the power jack footprint on the right-hand side of the board.


Bend the leads...


Drop in place, bend the leads over on the backside...


Solder and clip!



4. Other Current Sense Resistors

Getting the hang of this? Great!

Next, let's install the 16 ohms (color code: Brown/Blue/Black/Gold) current sense resistors. These come as a segment from a large spool. Just pull them straight out and straighten the leads as needed.


Installation procedure for these is exactly the same as the resistors above. These will go in locations R11, R13, R15, R17, R19, R21, and R23 (in a big line across the middle of the board). Here's what they look like when installed:


5. Base resistors

Next, locate all the 1k ohm resistors (color code: Brown/Black/Red/Gold).


Install them into locations R12, R14, R16, R18, R20, R22, R24, and R26 (again, in a line across the middle of the board, right above the resistors we installed in step 4). You may have noticed that we didn't use all the 1k ohm resistors available. Thats okay, we'll use the last one in step 6!

6. LED Resistor

Last resistor, we promise!

It's a 1k ohm (Brown/Black/Red/Gold), exactly like the resistors in step 5. This resistor goes in location R27, between the large IC footprint near the top-middle of the board.

This is what your board should look like now:


7. Zener Diodes

Be careful with these - the resistors we installed above could be installed in any orientation, but not so with these diodes!

Look at the board for a second, and locate locations D1 and D2 (upper left-hand side of the board). Notice the thick white lines inside the footprint, on the left of the box?


Take a look at the diodes. See how they have a black line wrapped around one end of the glass cylinder?


These are polarity markings - when installing these parts you need to match up the black line on the diode with the white line on the footprint, like so:


Installing the diode backward could cause your Big Digit Driver to be damaged when you plug it in! Double check the orientation of your diodes before you solder them.

Other than keeping an eye on the polarity, installing the diodes is pretty much the same as the resistors we installed above.

8. SPI Header Ground Connections

Next, we will install the jumpers that connect the Big Digit Driver to the SPI ground line. For this we'll use zero-ohm 'resistors' - they have no real color code, they look like resistors with a single black stripe instead of five or more. They're not really resistors either (well, they do have a resistance - like all wire - but it is very low!) and each could actually be replaced with a length of wire.


We'll be installing them in locations JP5 (upper left-hand side of the board, just below the diodes) and JP4 (upper right-hand side of the board). Here is what they look like when installed.



Please note that we leave locations JP1, JP2, and JP3 empty!

These locations are the Schmitt trigger bypass jumpers. The Schmitt trigger is used to 'boost' the signals that run down a series of drivers and helps eliminate synchronization issues due to noise and attenuation.

If, for whatever reason, you don't want to use the Schmitt trigger you will need to install four jumpers in locations JP1, JP2, and JP3. Ninety-nine percent of our users should NOT install these bypass jumpers!

DO NOT use both the Schmitt trigger and bypass jumpers at the same time! Doing so shouldn't cause any damage, but the behavior of displays down the line will be strange and unpredictable.

You may note that photos later in this instruction set have these jumpers installed! This is for illustrative purposes only! Unless you have a good reason to bypass the Schmitt trigger, do not install these jumpers!

9. Shift Register and Schmitt Trigger Sockets

Next, we will install the sockets for the ICs.


There are two IC footprint (U9 and U10) and they are slightly different sizes (U9 is 16 pins, and U10 is 14 pins). Make sure you have the right socket for the right footprint! If it doesn't drop in, or if the socket doesn't seem to cover the entire footprint, double check that you have the correct socket.

Take a look at the board - notice the 'notch'/semicircle on one end of the footprint outline? This is a polarity marking. Match this mark with the notch in the end of of the socket.


Drop the socket into its respective footprint and flip the board over (you can either hold the socket in place or use the surface of a table/desk/bench to hold it in place). Now there is a bit of a trick to getting the socket flush the with the board. First, we bend over two pins in order to hold the socket mostly in place - notice how the bent pins are on opposite rows and opposite ends of the footprint.


Then, solder TWO pins - either the two bent pins or the pins directly across from them. Flip the board over and check that the socket is flush with the board. If not, apply light pressure to the socket on the 'high' end and reheat the solder joint. The socket should 'snap' into place. Remember to give the solder a moment to cool before you take your hand away!

At this point you can solder the rest of the pins. Trim what excess you can - it'll save wear-and-tear on the insulator when you mount the display.



Then repeat for the other socket.


10. Digital Control Transistors

These transistors are little black cylinders with one side flattened and have three leads coming out one end. They should come attached to a strip of paper tape, but double check their markings just to be sure. They should have the number '2N3904' on the flattened side of the cylinder.



We will be installing these into locations Q1, Q2, Q3, Q4, Q5, Q6, Q7, and Q8 (in a line across the middle of the board). Note that orientation matters! Take a peek at the board. See how the footprint is a circle with a flat side? That flat side matches up with the flat side on the part itself (the side with the text on it) - in other words, the flat side of the part should be facing toward the top of the board.


You may have noticed that the transistors also come with pre-bent leads - all you need to do is cut them loose from the paper strip and drop them in place.

11. Current Regulators

These will look similar to the transistors in step 10, except that they will have the number 'LM317' on them, do not come attached to paper tape, and do not have their leads pre-bent.


Install these in a line just below the transistors, in locations U1, U2, U3, U4, U5, U6, U7, and U8. Once again, orientation matters! Be sure to line up the flat side of the regulator with the flat side of the footprint on the board (the flat side should be facing toward the top of the board).


12. Five Volt Linear Regulator

This part will look look identical to the current regulators in step 11, except that they will have the number '750L05' on them.


Make sure you have the right part! There is only one voltage regulator and it will look very similar to the current regulators used in step 13. We will install this part in location U11 (upper left-hand side of the board, look for the words '5 Volt Regulator').


Like with the transistors in step 11, orientation matters! Again, line up the flat side of the regulator with the flat side of the footprint on the board (the flat side should be facing toward the top of the board). Getting the leads into the proper holes may prove a bit tricky - you may want to pre-bend them yourself before attempting to install the part.



13. Linear Regulator Output Capacitor

This part will be a little blue blob with two leads coming out of it.


We'll be installing it in location C2 (look in the upper left-hand side of the board, under the '5 Volt Regulator' label). Don't worry about orientation.



14. Other Capacitors

These parts will be yellow brown blobs with two wires coming out of them, and come on paper tape (similar to the tape the transistors came in).



These will go in locations C1, C3, and C4. Orientation doesn't matter with these parts, so you can just drop them in.



15. Power Indicator Lamp

The Big Digit Driver board has a power light. Nothing fancy, just a yellow LED. It goes in location LED1 - between the two IC footprint in the upper middle of the board.

Note that orientation matters for this part: the long lead goes into the square hole.



16. Screw Terminal Power Connectors

These are the green plastic blocks with screws in the top of them. You can install them in any orientation, but it's probably better to install them with the wire slots facing outwards.


These go in locations J2 and J3, on either sides of the board.



Getting them to stay in place when solder is a bit tricky - we recommend placing the connector pins-up on a flat surface and laying the board onto it.


Only solder one pin at first. Check to make sure the connector is lined up correctly before soldering the other pin.



17. SPI headers

These are also a bit tricky. The SPI header is a two by three block of straight pins. These should drop right into locations J5 and J6, on the left-hand and right-hand side of the board. Solder one pin on the back and then flip the board over. Make sure that the header is perpendicular to the board and the black plastic block is seated on the board. Once you're sure the header is seated properly, solder the rest of the pins on the back.



18. DC Power Barrel Jack

This part goes in location J4, on the right-hand side of the board.



Use the solder sparingly - just enough to tack it down. Flooding the holes may cause solder to bridge the gap between the holes and cause a short circuit.




19. Insert Shift Register

The shift register is a 16 pin IC that has '74HCT595N' on it. It is the top chip in this photo:


You may need to fiddle with the pins a bit to get it to plug into the socket at U9. Carefully pressing the side of the pins against a flat, hard surface is a good way to do this.

Orientation matters with this part! See that notch on one end of the IC (on the left end in the photo)? That should line up with the notch/semi-circle on the IC footprint on the board.


20. Insert Schmitt Trigger

The Schmitt trigger is the 14 pin IC that has 'SN74ACT14N' written on it. It is the bottom chip in this photo.


The Schmitt trigger and it goes in location U10. Like the shift register, orientation matters: the notch on one end of the IC (on the left end in the photo above) should line up with the notch/semi-circle on the IC footprint on the board.


21. Display connector

Almost done!

This part is the huge blue card edge socket with pins at a right angle. It goes in location J1.



Make sure you put it in the correct side of the board! The socket actually sits on the back of the board, not the front (i.e. not the side with all the other components). The connector faces 'upwards', toward the top of the board.


Make sure the socket is completely seated on the board before you solder it down. Clip off any excess wire.


22. You're done with the electrical stuff!

Good work, the board is finished. Now lets get it attached to a display, eh?


We'll be installing an insulator to prevent the Big Digit Driver board from coming on contact with the display and causing a short circuit. We'll also be attaching metal standoffs to the back of the display for mounting purposes.

23. Attach The Display

The driver board slides onto the back of the display. Plugging the display into the edge connector should do a pretty good job lining up the mounting holes.


24. Delrin Insulator

The plastic insulator goes between the driver board and display. Note that the insulator has two holes - these line up with the middle pair of holes on the driver board (specifically, the holes that are slightly farther from the edges than the other four and line up with the mounting holes in the display).


Be careful with the insulator! It is resistant against punctures, but is actually rather brittle. Don't twist or warp it unnecessarily.

25. Board Spacers And Standoffs - Bottom of Display

To secure the driver board to the display we use a 1/8 inch (3.2mm) thick nylon washer, 1/2 inch (12.7mm) long 6-32 screw, and a 3/4 inch (19mm) tall threaded standoff. On the bottom corners, pass a screw through the display board, insulator, washer, and driver board. Screw a standoff onto it.


26. Board Spacers And Standoffs - Top of Display

To match the height of the stack-up on the bottom of the display, use the 1/16 inch (1.6mm) thick nylon washer, 7/8 inch (22.2mm) tall threaded standoff, and 3/8 inch (9.5mm) long 6-32 threaded screw. On the top corners, pass a screw through the display board and washer. Screw a standoff onto it.


27. Really done this time!

That's it, you're completely done! Hang it on a wall and plug it in! The kit includes four extra 1/2 inch 6-32 screws for mounting purposes.