Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

LED Display circuit design

Not open for further replies.


Newbie level 4
Sep 9, 2009
Reaction score
Trophy points
Hot Springs AR
Activity points
led display circuit

I am attempting to replace the obsolete gas 7 segment displays on my pinball machine with seven segment LED displays. The system now uses MC14543’s (+5 Volt ) for the BCD decoders and 7180’s for the segment driver for the gas display. It also uses UDN6184’s for the digits.
My plan, as suggested by another pinball enthusiasts, is to replace 7180’s with ULN2803 (+5 V to pin 10 and Ground on pin 9) and replace the digits drivers UDN6184’s with UDN2981’S (+5 on pin 9 and Gnd on pin 10).
The gated pulse outputted from pin 9 (segment “a”) of the BCD decoder MC14543 is +5V in the on condition and o V in the off. The same pin 9 is direct wired to pin 8 of the ULN2803 and it’s output is on pin 11. This output is tied through a 330 resister to the “a” segment of a common anode LCD display. The common anode is connected to pin 18 of the UDN2981.
Other than the fact that that the display doesn’t work, the signal output of the ULN2803 is very weak. It appears to be only about a tenth of volt. Can anybody out there help me? Will the components I have mentioned above even work with them hooked up as I have described?
Thanks, Gerald

led circuit design

Can you sent the actual schematic and modified schematic. we can check the response with that circuit only. and guide you for modification.


led circuit schematic


Changes: 1. Replace U2 (7189) with ULN2803
2. Disconnect U2 pin 10 from –100 V and connect to +5 V
4. Replace U4 (UDN6184) with UDN2981
5. Disconnect U4 pin 10 from +100 V and connect to Gnd.
6. Disconnect U4 pin 9 from Gnd and connect to +5 V
7. Substitute LED (Common Anode) for the gas display

Note: I can’t find the part number on the LED’s but I
have bench tested them by putting +5 V on the Common Anode
and Gnd to each segment through a 330 Ohm resister
and they work.



Are you going to connect all chips directly without any R/C/Regulator?

led display circuit design

Well, perhaps that's the problem. Do you have any suggestions?

Added after 4 minutes:

Since the original circuits were wired directly, it would be difficult to add components. I had hoped to use chips that would interconnect like the original.

Added after 9 minutes:

beneath the schematic under the heading changes: In step "1.", I refer to U2 as a 7189. This is a typo and it should read "7180"

Added after 46 minutes:

I have discovered several errors in the original schematic. I have made corrections on the following attachment. I'm sorry I was so careless.

udn2981 schematic

geraldeubanks said:
Do you have any suggestions?
Hi Gerald,
both your ULN2801 & UDN2981 are Darlington drivers, and so have saturation voltages of about 1 to 1.5V each. Your LED-Display - if it's a red one - needs at least 1.5 ... 1.8V, so an overall voltage supply of 5V seems a bit scarce. Try and supply the UDN2981 with a higher voltage, say 8 ... 10V (pin 9).

Also, the MC14543 decoder @ 5V op. voltage supplies only (a min. of) -0.44mA output current @ Voh (high level), which also might be a little bit scarce. If the first solution (above) doesn't help, I'd suggest to increase its op. voltage, too, (pins 1 & 16) - could be the same supply voltage as that of the UDN2981.

Cheers, erikl

led circuit

Thanks erikl, I appreciate your input. First, let me explain how the displays work. The components are all located on the master display board along with two on-board digits for "number of games credit" and two more digits for "ball in play". Then there are the four remote "player score" displays each consisting of 6 digits. Although not shown on the schematic, there are are actually two ULN2803's that feed the seven segments of data to the two on-board 2 digit displays and the four 6 digit remote displays. There are actually four of the UDN2981's that drive the digits strobes (Units, ten's 100's, etc).
I've done a little more checking and I have just found out that if I disconnect the common emitter (U4 Pin 12) and connect it directly to + 5 V, the LED lights up the "0". I had expected the LED to be garbled even if it lit up since there are four more displays that the 7 seg. signals feed to. Then it dawned on me that since it's the lowest digit that all the other displays might be "0" also and so, after checking, I found out they were indeed. So now it looks like the problem is with the UDN2981's.

a pinball LED ?

try this one :

**broken link removed**

You have to reduce the 330ohm resistors by 33 ohm.

Thanks tinhead, you found another typo: they are 33 Ohms. The link you sent is the site where I got the idea. It just doesn't seem to work. To make matters worse, the author doesn't show a schematic so you're never sure of what your doing.
The ULN 2803 segment gates (from decoders) provides a solid Ground to the active segments but the strobes or gates (for each digit) that drive the UDN2981's doesn't seem to get through as a full 5v. On my old oscilloscope, it seems to be less than a volt. If I simply disconnect the common anode from the UDN 2981 and tie it to +5v, it works great but of course it only displays the right number if all digits are the same.
If I'm looking am my scope right, I'm pretty sure the strobes are positive pulses and the driver should provide a full +5v pulse to the common anode of the display. Does anyone know if a UDN2981 will work as a driver for a Common anode LED Display in this configuration?

Not open for further replies.

Part and Inventory Search

Welcome to