A few years ago, I bought a number of Flip Digit displays, that were originally used on old Petrol pumps here in the UK. They were from Gilbarco brand pumps. I de-soldered the flip digits from the Gilbarco circuit board and tested each of the 7 magnetic coils, to ensure I got a stock of fully working units. Some were burnt out and some got damaged during de-soldering. I ended up with a few dozen units, enough to make several clocks. Here is one I made.
This was a sandwich design, with the main electronics hidden behind the displays:
but a bit messy with a lot of wires to connect the display board to the controller board.
The main issue, however, was that I used the original driver IC's that were designed to power the flip digit displays, FP2800A. These are almost obsolete, but can be found, however are now expensive, and my clock design needed 2 IC's per clock. This was adding £20 ($25) to the cost.
I've recently revisited these clocks and I have build several more of the above type, but now I have depleted my stock of FP2800A Drivers. I started to think about alternatives, and found a few good ideas on t'web, https://coytbarringer.com/flip-display-clock/ is a good example, creating H-Bridge circuits using N and P channel Mosfets. There are others using H-bridge Motor drive IC's etc.
I have decided instead to go old-school and continue in the theme of the flip-digits (which make a noise as they move), so what better to complement the clicking noise of the segments moving, but the clicking of a few relays? !! Yes, I'm going to drive the flip digits using relay switches.
Driving ChallengeThese flip digit displays are not easy to drive. each segment is controlled by a coil. A voltage (16-18v) is applied momentarily in one direction to "set" the segment. the segment remains "set" with power removed. 1ms is enough to flip the segment. The "reset" the segment, the voltage must be reversed. Therein lies the problem. How to reverse the voltage easily.
There are lots of ways to do this, and very easily with relays. I decided to multiplex 4 Displays and then multiplex the 7 segments on each display to each other. This cuts down on the number of circuits (relays) needed. The impedance of the coils is low, so blocking diodes are needed to stop the signal back tracking through other coils in the multiplex array.
Here is one coil:
Each Segment will receive 18v when it is required to be set or unset. (MV = Mid Voltage). I can now effectively Set and Unset quite easily. By applying +36V to D1-HV to Set or by applying 0V to D1-0V to Unset. Note D1-HV and D1-0V will never be both on at the same time (as would cause a short +36 to GND via the diodes).
Each of the Segments is multiplexed across the 4 displays, and each display has it's own 2 control lines D1-HV and D1-0V, D2-HV and D2-0V, D3-HV and D3-0V, D4-HV and D4-0V.
So that's 7 segments and 8 control lines = 15 relays needed.
I think it may look nice to put all the electronics on a single board with the 4 displays at the top.
My chosen processor is ESP-322 and I just love using a devkitV1 board, which can be plugged in as a daughter board.
For a clock, I will also use a plug in off-the shelf RTC clock module board. (These are cheaper than adding the components myself or by the PCB factory).
I've done a few other clocks recently using TFT LCD screens and for those I added a Temp and Humidity Sensor, using DHT22 module, so I may as well add that too.
Also, a simple piezo buzzer is handy to have for alarms, etc. They are cheap and easy to add and control.
One thing I missed on my original flip clocks was a second indicator, so for this project I will add a couple of APA106 addressable RGB leds to show a colon between the hour and minutes.
So, current state of play is looking like this:
4 flip-digit displays at top, 2 leds in the centre. 16 relays underneath with control electronics underneath each one. At the bottom is RTC, ESP32s, Buzzer, DHT22, 2X 595 Shift register IC's and 2 off the shelf Buck booster boards to provide 18v and 36v.
I've added LED's to indicate when each relay is activated. However, I have made it so that these LEDs can be switched on or off globally (They are controlled by the 16th relay (I added that as it just looked nicer, so now I found a use for it!).
Schematic so far:
Next stage is to get the PCB's built. I will get the SMD components installed in the factory. That's the LEDs, Resistors, Diodes and Transistors. I will add the relays, flip digits, daughter boards, 595SR's, temp sensor and buzzer.
Now sit and wait for the PCB's to arrive..........zzzz
I had a sudden thought, would this be too top heavy? The magnetic flip displays are quite heavy and it may fall over. So..... another PCB idea:
Not sure which I like best, but maybe this horizontal one is the way to go?
OK 3/12/23 - I ordered a horizontal board. Wait now until it arrives.... BUT of course I can't wait and do nothing! So I started to look at a flip dot display I bought a while ago:
Its a hanover 84X7 flip dot display. I opened it up to reveal a controller pcb behind the flip dot displays (of which there are 3, each 28 X 7 matrix. I will separate them to make 3 clocks.
There is control logic chips on each display board, so I'll try to make use of that. Some reverse engineering is needed.
The first board arrived and I started making it up, I added enough components to test most of the operation out and it appears to be working, although I had to mod the circuit a bit to stop some powerup issues.
I am also getting some interference on the Shift registers when the relays trigger (strangely both on the 36v and also on the GND). Probably made a schoolboy error in the placement of the 595 Shift registers so far away from the ESP32, across the Higher voltage PSU's. Anyhow, I can deal with that for now within the software, and I will improve the design for the next board. I've got the next board design started already:
I have moved the shift registers to be close (underneath) the esp32s, which sits in a socket, so plenty of room for 2 ics underneath!
With regard to the flip dots displays, I will document them in a separate project, but they are exciting too!
New V1.0 Board has arrived, I made up a board with just enough components to test it out.
The 595 Shift registers are hidden underneath the ESp32s board
I put in a jumper to allow the Shift Registers to run at 3.3v or at 5.0v. You can see it in the above pic with the blue jumper. (I also added power jumpers to the 2 buck power supplies, so these can be easily disconnected, which is useful for code debugging, we don't want to inadvertently power the flip digit coils too long.
I did some initial testing and at first all appeared well, but then I noticed still occasionally getting "interference". I had the "595" Shift Registers powered from 3.3v, I switched them up to 5,0v and the issue has disappeared! Looking at the 595 specs, they should (and have for me in the past) work fine at 3.3v, but hey ho. I tested a lot more to ensure the ESP32s outputs are correctly driving the 595's at 5.0v, and all looks fine.
I'll make up a full board and do some more coding.
I made up a full board. now getting the interference back. Damn. Think I'm going to go solid state rather than the relay option......
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