Using breadboards, I often build project prototypes in stages as it makes sense to prove that each section works before moving on. I’ve just started another project and was confused as to why a part of it wouldn’t work. At first I suspected faulty components but ruled that out. After using my logic analyser to check the data flowing and convincing myself that the software was ok I was at a dead end. It was by pure accident that I found that one of the sockets on my breadboards is faulty.

This is one of my oldest breadboards and has given me many years of service, but it looks like it may be time for the scrap heap.

The video shows that the spring loaded contacts beneath the hole won’t grab a piece of wire.

I’ve never had a socket fail before like this, so, in future this is something that I will keep my eyes open for.

My central heating has been on the blink for a while and the other night it finally died all together. I’m not one for messing around with gas appliances so called in the local gas engineer, one that I’ve used in the past and trust.

After poking around for a while he decided that the fault was on the circuit board, and they are VERY expensive to replace. I asked if he could remove the board so I could have a look; perhaps there was something obvious.

The picture shows a view around one of the connecting socket terminal pins. You can see that the pin has been making intermittent contact with the solder on the track and sparking (debris on the left of the pin). This also explains why a quick “thump” on the main unit would often bring it back to life for a while.

I took the board and using my inspection microscope found several other cracked solder joints.

It took me 10 minutes to inspect and re-do every suspect joint. There was also a resistor that had been overheating so I replaced that whilst I was at it.

All the bad joints were at the same end of the board and I suspect that this part of the board suffers from thermal shock due to its location under the actual heat exchanger. Whatever the reason, once the board was reinstalled everything started working normally and it saved me a £250 bill for a new board; a board I hasten to mention that has around £10 worth of components on it.

In the end when I asked how much for the work he only charged me for the call out as in his words, the repair was a “joint effort”.

It’s been a busy couple of days playing with the Nixie Clock but it’s been well worth it.

The firmware for the PIC is completed and running and the main logic board that contains the PIC, RS232 interface, Audio Amplifier, HT and +5v PSU is complete and working.

I had to make a few “modifications” to the wooden case as the display board PCB was slightly too wide and the 7-way Molex connector was catching on the side, preventing the board from sliding into place. Thank heavens for electric files !!

The chap who will be receiving this is a bit of a change freak and loves things that are configurable, so every parameter can be configured by hooking to clock up to a dumb RS232 terminal.

There is provision for two temperature sensors to be connected, and you can set alarms if minimum or maximum temperatures on either channel are exceeded. Alarms in either flashing coloured LED’s and/or an audio alert (frequency configurable of course).

There is provision for the CPU to power down the Nixie tubes during a specified time window. This could be useful during the day when the owner will be at work and should help extend the life of the tubes.

The on board RTC (DS1802) can have its date / time  set via a simple command over the serial port, and is also responsible for trickle-charging the on-board NiMH back up battery.

All that’s left to do is fit everything into the case and possibly make a few alterations to the firmware. I’m using just under 25% if the available program space so lots of room available for additional features. I’ve also added a 8-way Molex connector to the top of the PCB that brings out +5v, 0v and the remaining unused I/O pins from the PIC; this will make hardware expansion simpler when it’s required; and it will be required at some point.
I've also marked all the connectors so that when enquiring hands take the darn thing apart, it can be put back to gether... that's going to happen at some point as well I suspe

Many years ago, I constructed a Nixie tube based clock for a friend, and whilst it worked, it never really worked well. It had an MSF decoder so could automatically synchronise itself from the radio clock at Rugby in the UK (since moved to a new location). However, with all his computers, PSU’s and other radio equipment, the MSF receiver never worked properly. Also, I never gave the design that much thought. It required a cumbersome power supply arrangement to drive the logic and provide the high voltage for the Nixie tubes. I used a PIC microprocessor to control everything but I ran out of program space so the project never reached its full potential. I also ran out of time and enthusiasm which didn’t help.

However, since I’m now officially on vacation for a week or so, I thought it would be an interesting project to re-do, and this time, do it properly.

This time I’ll approach the project in a slightly different way. Instead of one large PCB attempting to house everything, I’ve gone for a modular approach. This will make hardware upgrades a lot simpler in the future for one thing. Also, it allows the project to be constructed and tested in blocks and will cut down on re-work time if a fault is found in a specific area.

The picture below shows the major building blocks of the project.

At the rear is a small PCB that contains a simple High Voltage inverter. This is really just a prototype and will be re-worked into a complete PSU board for the final build. It’s running from a 9v DC output from my bench PSU and producing the 150v DC to drive the Nixie tubes. The output voltage is adjustable via an on-board variable resistor.

The large board in the middle is the completed display board. This contains a 64-bit shift register constructed from eight 74HC595, 8-bit shift register ICs. There are also 50 x MPSA42 High Voltage driver transistors on the board to control the Nixie tube cathodes; all the tube Anodes are connected to a common HV rail via 47K resistors. There are also 10 x LED’s mounted in strategic locations at the front edge of the display board and these will be used to display operating mode; date, time, temperature etc.

In the picture foreground you can see a small breadboard that is currently home to an 18F25K22 PIC  on a carrier PCB, and various connections to PIC programmers, and my Shift Register monitor device (described in previous blog entries).

The completed unit will contain a Maxim DS1302 Real-Time-Clock chip with battery backup, and a serial port interface for unit setup. There will be support for two temperature sensors (one internal, one external) and temperature / date/time alarms.

The possibilities are almost endless and I’m really only constrained by the amount of code I can squeeze into the PIC, which should be substantial as this PIC is a far cry from the previous one I used with only 1K of program space.

The advantage of using the 74HC595 IC’s becomes apparent when you realise that you only need a total of five CPU I/O pins to control the shift register chain (and you can do it with less if you really want to). This leaves plenty of I/O pins for other devices. Also, if an upgrade to the display board was ever required, the shift register chain can be lengthened without impacting on the rest of the hardware.

If you need to create a project that has a lot of inputs or outputs, the 74HC595 and 597 IC’s are perfect.

I’ve been wanting to experiment with PIC’s and making sound effects. After thinking for a while about several ways I could do this, I realised that the one thing I didn’t have was a decent audio amplifier for the bench.

A modest 2 to 5 watt unit can be constructed using a single IC and a handful of components. However, I thought that if I’m going to the trouble of making one, I may as well do it right.

The picture shows the front panel of the final unit. It contains a pre-amplifier, a three-band graphic equaliser and a power amplifier delivering around 5 watts maximum into 8 ohms.
There are some switches on the front to allow for bypassing the pre-amp and graphic equaliser and some other basic functions.

The unit contains its own internal mains power supply and as you can see, fit’s in a nice small foot-print. There is a socket on the back of the unit to drive an external speaker.

The unit was constructed with what I had available to hand; including the case, and made a nice weekend project.

If people are interested in the design, I'll publish details.

Another new piece of equipment I’ve invested in is an anti-static bench mat from Rapid Electronics.

http://www.rapidonline.com/Tools-Equipment/Killstat-bench-matting-77353/?sid=61428607-0425-4fc9-abd6-1b15561bd6e4

Now, I’m not one of these static paranoid people that seem to flourish in the electronics world. I’ve never damaged anything with static; that I know of, but I’ve always taken “sensible” precautions like wearing an anti-static wrist strap when poking around with sensitive components.

However, when I built my workshop, I opted for the cheapest worktops that I could find in the required quantity, which turned out to be in a nice grey granite affect. Unfortunately, I’ve started to notice that as my eyes are getting older (and I now wear glasses for the first time); it’s harder to see some components when placed on the worktop.

I’ve opted for a mat in light blue measuring 900 x 610mm and it’s just about perfect for the job. It provides good colour contrast for components and I even managed to find a couple of SMT capacitors that I dropped; the little buggers blend in perfectly with the grey/black granite affect worktops and are almost impossible to find.

They are available in several different colours and sizes.

The above picture was taken from approximate head-height and shows the problem I have with components and small parts placed on the original work-top surface.

The organisation I work for have many buildings and most contain at least one computer or equipment room. In one particular building, the computer room air-conditioning is temperamental to say the least and because people don’t often need to go in there, one more than one occasion the air-conditioner has tripped and it’s been a while before anybody’s noticed the tropical heat wave going on in there.

Yesterday I designed and built a temperature alarm system. It’s nothing too clever, just three large LED displays in a box, with an 18F25K22 PIC running everything. A Dallas 18B20 sensor has been mounted in a small box and is connected via a piece of three-core cable to the main display unit that can be mounted outside the room.

You can set the maximum allowed temperature and this is stored in the PIC’s EEPROM.

If the temperature rises about the pre-set maximum, the display flashes and there are a couple of 5v buzzers inside the main unit to help attract attention.  There are three additional indicator LEDs housed within the main display unit as well. One is a green LED that flashes indicating all is well, a yellow LED that indicates that the unit is in setup-mode (started by powering on the unit whilst depressing the external push-button switch mounted on the side), and a red LED that indicates the temperature alarm has been tripped at some point. Depressing the push-button switch once clears this indicator and the complete unit is powered from a small 9v plug-in mains transformer.

Inside the unit there are two PCBs. One is the top display board, and the other is the logic and PSU board.

If people are interested I’ll publish the design details for this unit.