Help with checking schematic and understanding what resistors to use


I'm going to create a Word Clock after being inspired by the Biegert and Funk Qlocktwo, so after some googling I have found some guides on how to do it. But the problem is that most of them is made by creating their own arduino board and I decided to not do that but instead buy a Arduino Uno. So now I need help to check if I have understood the original schematics correctly when I redid it for a real Arduino. I can also add that this is my first ever project using an Arduino.

This is the schematics I decided to copy:

and this is my version of it:

Also concerning the LEDs I'm not quite sure which resistors to use, so if anyone could tell me which ones to use/calculate which ones to use it would be much appreciated. I think I will power everything with either 9v/12v not sure which one would be most suitable for this project. The LEDs are Size: 5mm Forward Voltage: 3.2-3.4V Current: Not sure since the package didn't say anything. But my guess would be 20mA

As far as I have understood I calculate the resistor value like this: (Power Supply Voltage - LED Voltage)/LED Current But I'm not sure how to calculate if there is more than 1 LED. They will be connected like this:

I would assume that it wouldn't change considering they are connected parallel. As I said previously it would be much appreciated if someone could tell me if this is correct or wrong.

Thanks in advance

Click Reply, then Additional Options, Browse to your diagrams/whatever and Attach them. I can't get to

Your formula for LED current limit resistor is correct. If you have them wired in series, then: (Power Supply Voltage - LED Voltage - LED Voltage -LED Voltage -)/LED Current for 3 in series from a 12V supply. You will also have a transistor at the bottom of the string, you could subtract another 0.7V for say an NPN transistors Vce, or (Rds x LED current) for a Logic level P-channel MOSFET. Otherwise, your original formula for each LED in parallel - each needs its own resistor too. TPIC6B595 is a good shift register for controlling strings of LEDs. Output is rated to 50V, can continuously sink 150mA per output channel. I use up to 12 of them on this board for controlling 7-segment displays: You can see the Arduino functionality at the lower left, Serial programming is done via an off-board USB/Serial adapter.

Ok here is a pdf with the schematics.

Edit: Also added a file for Eagle

Word Clock.pdf (85.2 KB)

Word Clock.sch (248 KB)

Add a 0.1uF cap across each shift register from VCC to Gnd. Then, LED operation is as discussed earlier.

I’m quite new to electronics so I have to ask, what exactly does the capacitors do if I put them like that? I wanna learn as much as possible while doing this. :slight_smile:

Also added the update schematic, just incase I got it wrong where to put them.

Word Clock.sch (398 KB)

The shift registers can have large internal currents as things switch - the caps provide the current needed to keep things stable. Schematic looks good.

Some use pin 10 Q'S for output to next D. Others use pin 9 QS for output to next D.

I have used pin 10 in the past, but I noticed that you use pin 9. What is the difference ?

According to the datasheet: "Two serial outputs are available for cascading a number of CD4094BMS devices. Data is available at the QS serial output terminal on positive clock edges to allow for high-speed operation in cascaded systems in which the clock rise time is fast. The same serial information, available at the Q’S terminal on the next negative clock edge, provides a means for cascading CD4094BMS devices when the clock rise time is slow." But I still don't understand what is better.

Try both ways, see if there is a difference. I think the former will work better.