So with my clock sitting idle on the wall it is time to come up with a design that will send it periodic pulses, once per minute, with +24V and then -24V on alternate minutes…
I really want this to be pretty stand-alone and I don’t want to faff about with WiFi so that eliminates NTP, I do not want to be near a window so that eliminates GPS so that leaves me with either a free-running clock or radio frequency receiver. My recent experiments with the MSF radio signal, which is really hard to receive around computer equipment in London, led me to DCF77 which has similar problems but has better signal strength. The MSF signal moved further up north a few years back and the transmission power was also reduced during the move so surprisingly DCF seems to work better here.
I plan to use some off-the-shelf parts so I’m not going to be clever in the first iteration at least… I need a controller, a radio clock module, a 24V supply, an H-bridge to send 24V (and reverse the direction each minute) and for fun an LCD display and a couple of buttons to change modes etc. Modules are cheap as chips from China so I’m just going to attach a few things together and maybe later reduce the design and make a PCB.
Here’s the Radio Module I bought from PV Electronics (http://www.pvelectronics.co.uk/index.php?main_page=product_info&cPath=9&products_id=8) (who also make nice NIXIE kits)…
I put it in a cardboard box for now, at the end of a bit of cable so I can move it away from my computer monitor. RF Interference from computer equipment completely prevents reception using these receivers, the MSF 60kHz or 77kHz DCF signal being too close to the timebase in monitors etc.
For the display I have a 16×2 LCD display with Hitachi controller in the junk box, for the controller a cheap chinese UNO clone and then a H-Bridge module and DC-DC converter (5V to 24V) from EBAY. Three chunky buttons complete the hardware.
and inside the box…
The LCD display wires straight into the Arduino UNO, with just a small 10k potentiometer to allow the contrast to be varied, it is a 5V display so compatible with the 5V Uno clone. The DC-DC converter is just a quick and dirty way to turn 5V into 24V and the H-Bridge a simple (rather over the top way) of being able to reverse polarity to the clock. (The radio module is located in the cardboard box for present a metre or so away).
It’s almost unbelievable how cheap some of these parts have become. The L298N H-Bridge can be had for around £1.50 from HK on Ebay, the XL6009 DC-DC converter for around £1 and with a bit of shopping around the Uno clone is about £2.50. So hardware cost is around £10 plus the radio module.
When choosing a Arduino Uno clone take time to try and find one with a crystal oscillator (shiny metal can next to processor) since the DCF decoding software needs a reasonably accurate local clock and some of the ceramic resonators in the Uno clones are terrible (2000 ppm). If you are unlucky enough to get one with a ceramic resonator then with a bit of careful soldering you can easily replace the resonator with a crystal.