Nixie Voltmeter Clock


I recently acquired a vintage nixie tube multi-meter on eBay, with the not so original idea of converting it into a clock.

However, rather than hack around with the internal circuitry, I decide to see if I could keep the meter intact and use the existing voltmeter functionality to display time.  The basic idea is to send the meter a voltage corresponding to the time, so e.g. 12:45 becomes 12.45V. The meter has a 20V range which displays a fixed 2 decimal places which suits my purpose just fine. Unfortunately there is a hard limit to 20V on this range so a full 24 hour clock is not possible, but a 12 hour clock and maybe a 20 hour clock are.

The high level schematic is:

RTC -> Arduino -> DAC ->Multi-meter



Each minute on the display corresponds to 10mV so the DAC must be at least this resolution. I used an AD5501 which has 12bit resolution and can work at up to 60V. For my needs I used the 30V range which works out at 7.3mV per bit, just enough. This DAC also has an integrated voltage reference which reduces the component count. It comes in a TSSOP package so was soldered onto a breakout board for easier prototyping.

For the RTC I used a DS3231 module. This is accurate to about 1 min/year and has a battery backup. It can be controlled from the arduino over an I²C bus.

The Arduino runs a simple program which at every new minute gets the current time, calculates the required voltage for the time, offsets it by a calibration factor and sends it to the DAC. The program also accepts a few commands over the serial port to calibrate the display and set the current time.

Here is everything running on a breadboard:

Breadboard layout

For power, rather than tap into the internal multi-meter power buses I hooked up a simple 240VAC/5VDC module with a cheap Chinese boost converter to give 15V for the analogue power.

The meter has space inside for a battery so there was no need to miniaturise the component layout and I just wired the modules together on a bit of strip-board, fixed it to an insulating base and popped it into the meter.



Connecting various lab instruments to ethernet

I have an assortment of lab instruments that I’d like to control from a PC. They have a range of connection options: RS232, USB, GPIB, Ethernet and support the SCPI protocol. Decided to standardise on Ethernet.

The three Rigol devices all have Ethernet connectivity so I assigned them all fixed IP addresses via the front panels, plugged into the LAN, and then connected via telnet (putty) to port 5555.

Rigol DP832

Responded to *IDN? query OK

Rigol DS1054Z

Responded to *IDN? query OK

Rigol DSA815

No reply to *IDN? query 🙁 I suspected the DSA815 did not like the CRLF line termination from putty and sure enough, sending just a LF terminator (0x0a) from a simple test program worked OK.

The other two older machines did not have an Ethernet port, so I connected via two Ethernet/RS232 adapters.  I used the USR-TCP232-2 module from eBay. This doesn’t provide any hardware flow control so will need to bear that in mind for the PC control software.

The instructions were not the clearest but in the end configuration was straightforward.  Plug the device into the LAN and run the USR-TCP232-T24V5.1.0.1.exe setup program. Click “Search in LAN” and you should see your device in the list. Select “TCP Server” mode, give it a new IP address and set the subnet mask and default gateway for your router.  Set the module port to 5555 to match that used by the Rigols. Configure the RS232 port as needed.  Then click “Set selected item via LAN”.

Keithley 2015 THD

Set RS232 comms to 19200,N,1,1 from the front panel.  The 2015 has a female RS232 connector and needs a M/M serial cable (crossover 2<>3, 3<>2, 5<>5).  Set to no flow control for now. It also has the option of XON/XOFF flow control which might come in handy.

HP 33120A

Set RS232 comms to 9600,N,1 from the front panel.  This unit requires 2 stop bits. It also requires pin 6 (DSR) to be held high when hardware flow control is not used. The unit has a male RS232 connector and needs a M/F null modem cable (2<>2, 3<>3, 5<>5, 6<>6). On the USR-TCP232-2 module I soldered a jumper lead from the +5V power pin to pin 6. Does the job.