1960s HP Counter Nixie Tube Clock/BG Display

This is a project to make a clock-and in my case, a blood glucose display- from a vintage 1966 HP 5532A frequency counter. In my case, the counter didn't work, and I had to do some repairs. These initial photos are some of the repairs. This instructable will assume yours is working, and also that you have the ability and desire to set up and configure a Raspberry Pi and do some coding. The ability to safely solder is also a requirement. Due to the high voltages required to fire the nixies, extreme caution must be used, and the device should never be worked on while connected to power.

Supplies

Frequency counter

Soldering iron/solder

Raspberry PI zero W

120VAC 5V USB charger(may or may not need depending on counter model)

Optocoupled solid state relays for handling the nixie voltages (may or may not need depending on counter)

Python clock code

Small wire

Step 1: Figure Out How to Increment the Counter.

This step will vary depending on the counter you have. You may be even able to use an old multimeter or some other vintage "digital" equipment for the clock. They key is to figure out how the display works. In my case, I was able to download a technical manual from Artek manuals. Analyzing the schematic is beyond the scope of this instructable, but a basic knowledge of electrical/electronics theory is needed. In this case, I soldered a wire to the input lead and attached the other end to the GPIO of the raspberry pi. I used Python code to toggle the GPIO high and low and experimented to see what works best. I soldered a pull-down resistor (10K, I think) from the GPIO pin to ground to prevent 'floating'. I also cut the link from the 3rd decade counter to the 4th, and attached that to another GPIO pin so that I could increment the 1st 3 digits separately.

Step 2: Supply Power to Pi/Execute Additional Nixie Controls If Needed

I cut open an old 120VAC USB charger and connected it to the switched AC input of the counter, and soldered a micro USB cord to the charger output. Also, in this case, I wanted to control the decimal lights to indicate the blood glucose trend. They use 150VDC to fire, so I had to use optocoupled solid state relays soldered to the Pi. They are attached directly(with limiting resistors)to the headerless GPIO pads, which I used to signal the relays.

Step 3: Set Up Pi

You will need to set up your Raspberry Pi to connect to your WiFi, and load the Python clock script. Then you'll need to set it up to start on boot, by creating a .service file. In my case, I have my son's blood glucose displayed also, taking data from a local web server to display the value and trend. You could modify it to pull local temperature data (or sports score, or anything you want) and display it as well. You will have to modify the script to just show the clock if that's what you want. You can see in the script how it increments from 59 to 100 when needed, and in turn cycles the next digit to the left if necessary. You also may need to experiment with the timing of the signals to provide accurate display counts; I found that this device would only count accurately if the first 5 cycles or so had a small (.01 second per hi/low pulse) delay. After that, the machine can accurately count the Pi cycles as fast as it can produce them. In counting the first 3 digits, using an oscilloscope, I found that cycling the input from the -35V bus to ground, along with a 10K pull-up resistor to ground(pulling up because it was pulling from -35V) would create the proper waveform to increment the 10^4 digit by one each cycle. 2 of the solid state relays are used for that purpose.