# Analog+digital 12-hour clock circuit with LEDs: is this feasible?

#### xrandom66x

Joined Jul 21, 2015
13
Hey yall,

I have a personal project i just started on the other day which is a combination analog+digital clock. I am building it as a Christmas present for my dad. Before I go further, let me describe my expertise. I am a relative beginner to circuit design. I am a rising senior in college in Systems Engineering (like industrial engineering, not computer related really). I have taken some electrical engineering classes about electronics where we had to build a few modules like a binary counter with 10-digit display, debouncer, etc. Basic stuff. I also understood circuitry in Physics II very well. Aside from that, I don't have a ton of circuitry experience, but I am very good at problem solving and am too stubborn to quit if it's "too hard". Basically, all I'm asking right now is for someone who has experience to advise me if my idea is feasible or not, and to direct me to the right path, where to get circuit parts, different circuit components to look into, etc. Also, every bit of help is very appreciated, as I know you are helping at free will.

Ok, so design is still in initial phases and I am in the process of creating a rough diagram on MS Visio. Basically I want an analog clock above, a line below it that says "IT'S HUSKERS TIME!" (We're Nebraska fans), and a digital clock below that. All numbers, font, and a giant N will be backlit with LED's. I plan to design the clock circuit myself, just to add some personality and flair to it and have a clear back to see the circuitry. Here is the rough draft on visio so far:

Basically anything that is not black (besides my guide lines) will be backlit. I will design a clock circuit regardless (not too hard).

Now for my main question. All numbers on the analog clock will have a base luminosity. I think it'd be cool that when the hour hand is on a number, it is lit up brighter than all the others. As the hour hand moves to the next number, the first number will slowly dim and the next number will slowly brighten. So lets say that all numbers are lit, but dim (say, 20%). If it's 2:00, the 2 will be very bright (say, 100%). If it's 2:30, the 2 and 3 will be brighter than the other numbers, but not at full brightness (60% or so). The brightness will always be 20% unless the current time is within an hour of that number.

Also, along the same idea, I will have 5 dashes between every number for a total of 60 (like normal clocks). When the second hand passes by these, it'd be cool for them to light up as well, very similar to how I described the hours above.

All in all, is this feasible or am I a little too ambitious? how complex will the circuitry get? will I have to get special circuits?

Thank you in advance.

#### AnalogKid

Joined Aug 1, 2013
10,309
It is feasible, but it will take a lot of effort to get all of the pieces to play together. If the analog clock, the thing moving the hands, is a purchased analog clock movement, they keeping it synchronized with the digital stuff will be difficult. The first thing that comes to mind is using an optical sensor to detect when it is centered on 12, and use that signal to reset the digital clock counters. Having a bunch of LEDs around the face do something in time with the digital display is the kind of thing microcontrollers were put on this planet to do. It can be done with a ton of counters, decoders, gates, and drivers, or one PIC and a buttload of code. And with all of the display things you want beyond the basic 7-segment numbers, using a clock ship might actually work against you. Decoding the 60 minute LEDs from multiplexed 7-segment display signals will take more parts than doing the clock counters from scratch.

The basic digital functions (a numeric display, a 60-LED ring for seconds and/or minutes, and 12 LEDs for hours) are available on the innergoogle as assembled clocks and kits.

This all is doable, but a trick like dimming two hour numbers as a percentage of the minute value screams uC. Cool trick, lotta multiplexers or PWMs or stuff.

ak

#### djsfantasi

Joined Apr 11, 2010
8,904
Many of the analog clock modules can be hacked to get a 1Hz clock signal that will be synchronized with the analog display. This could be used to drive the digital clock. Hence, manually setting the two clocks to the same time and starting them should keep them in synch.

Your design has over 60 outputs, so you'll likely need shift registers and a micro processor.

#### xrandom66x

Joined Jul 21, 2015
13
Awesome thank you for the input. 2 questions: 1. How advanced is what I'm trying to do? Do you think I'd be able to learn and build this in 6 months based on the knowledge I posted in the OP? 2. would these components be expensive?

I hope to have a more complete design by the end of the day, if I complete it I'll upload it.

#### Brevor

Joined Apr 9, 2011
297
How advanced is what I'm trying to do?
I have to admit I winced when I read the list of features you would like.

As has been said keeping a mechanical analog clock movement synced to the display would be difficult.
If I were doing it I would try to modify the clock movement to take it's drive pulses from the digital control board. That way the control board would control the mechanical movement and keep it in sync. You would need to come up with some procedure to set the clocks to the same time initially.

Here are a few suggestions to make building this project less "painful".

1. Drop the idea to have the numerals fade in and out during their active hour, possibly just have them change from 20% to 100% in one step at the top of the hour and switch back to 20% at the end of the hour no fading. That will save a lot of programming hassles.

2. Your current setup will require a large number of outputs, maybe eliminate the 60 "minute" led's and/or the digital display.

Hard choices I know but engineering is always a compromise between cost, complexity and time to market.

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#### xrandom66x

Joined Jul 21, 2015
13
Sounds good. I also thought about a direct switch to increase brightness rather than a fade, but was curious to see how much harder it'd be. It'd look awesome, but probably not worth the effort.

I was also playing with the idea of only having the mechanical clock, but just having the digital for a quick glance. I may get rid of the digital display altogether.

#### xrandom66x

Joined Jul 21, 2015
13
Also I could just have the minute ticks as a constant led hooked up to the power supply. no circuitry needed really.

#### AnalogKid

Joined Aug 1, 2013
10,309
I like the fade idea, but a true fade or a multi-step crossover would take a lot of hardware and software. But a single-step fade would be just changing the timing of the seconds LED drivers to allow some overlap. For example, going from 2 pm to 3 pm - First 30 minutes, 2 only; second 30 minutess, 2 and 3 equal brightness. Whatever is backlighting all of the numbers, like a string of 3 or 4 LEDs per number, has one resistor direct to GND for the 20% background intensity, and two resistors each switched to GND separately. When both are on, that's full brightness for the first 30 minutes. Then one of them is turned off and one of the resistors for the next number is turned on for the final 30 minute. Each number has 3 intensities, background dim, 2nd half hour brighter, first half hour brightest. Timing for this is whatever is driving the hour displays, but time delayed 30 minutes. Easy to do in code or in chips. For an overall system wiring complexity, it doubles the hours drive signals from 12 to 24.

Even simpler is for the two numbers to be at full brightness in the 2nd half hour. Then there is only one drive signal to each hour backlight, and it is driven by OR-ing the two half-hour signals.

ak

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#### xrandom66x

Joined Jul 21, 2015
13
Ok so here is a question for something else. Id like the hour and minute hands lit up as well. After an exhaustive google search, I couldn't for the life of me find any LED clock hands. Do you think its possible I could make one by somehow supplying power to the gear part of the clock, which would power LEDs at the base of each hand, which could then light an acrylic rod that makes up the hand? I suppose a possible problem to this is 1 not messing up the motor, and 2 figuring out how to ground the LEDs... might have to compromise on this one. I guess I'm trying to make this clock into a lightshow, and its getting very complicated

Edit: Also thank you everyone for the advice on how to go about a fade, etc

#### AnalogKid

Joined Aug 1, 2013
10,309
Directly lit hands would be tough because the hands constantly turn in one direction. I suppose you could design very small slip rings, but this sounds difficult.

Normal clock hands are long things with points on one end and disks with a hole in it on the other. The disks press-fit onto the clock motor concentric shaft.

If your clock has a round decorative cover over the center part where the hands are mounted to the motor, an acrylic hand with a larger than usual disc would conduct light down its length. The light enters from a stationary LED on the face of the clock shining into the disc. This is a variation of a light pipe. The decorative cover hides all of this.

ak