sensitive 60kHz receiver: schematic part two

Schematic Part TwoShown on the left (click it to enlarge) is the circuit diagram for the receiver part of the clock – click it to make it readable or download it. This schematic is essentially a sensitive 60kHz receiver and ASK demodulator. You could use it as a simple receiver for other frequencies by just taking the signal at point 4 and adjusting the Inductor (antenna) and shunt capacitor to match the frequency you wish to receive.

The circuit should be fairly self explanatory. Firstly the power for the receiver comes from the PSU/decoder, and can be around 13-20V. A 7809 9v voltage regulator brings this down to 9v for our needs, there’s no need for a heatsink on the regulator if it’s the 1A type. As far as receiving the signal is concerned, the ferrite antenna and two shunt (parallel) capacitors adjust the frequency that they resonate at, according to this equation:

We can’t really change L, it’s set by the ferrite rod antenna (code LB12N from Maplin) Once you have found a C value to set f as 60kHz (I found C to be around 840pF), they will resonate and you’ll be receiving any 60kHz signal in the air. This signal is obviously tiny though, so we amplify it. The first stage of amplification is a JFET (2N3819). This has an extremely high input impedance, which is exactly what we want. The JFET’s gain is set to 10 by the 1k and 100R resistors. After a DC blocking capacitor there are two identical stages of op amp amplification. The gain of each op amp is set to -214 (the – indicates that there will be a phase shift, we don’t care about phase so that’s ok). Each op amp also acts as a low pass filter, attenuating frequencies higher than 72kHz, which helps us a lot.

After this we’re at point 2 in the schematic and we have a big 60kHz signal. This signal is turned into a DC level by the use of a low voltage drop diode (a germanium type would be ideal, silicon types e.g. 1n4001 are not suitable) and a 10n capacitor. The resulting DC level is then fed into a comparator, a low pass filter to weed out any fast spikes, then another comparator. The levels of each comparator can be adjusted. Simply adjust each 10k potentiometer until you get a decent signal. The LED will flash at 1Hz when a good signal is received.

The best way of setting the circuit up is by the use of an oscilloscope. If you live near Anthorn then you can ditch one of the TL081 op amps. The shunt capacitor value next to the ferrite antenna will need fiddling with to get a decent signal, and you may find that the resistors around the comparators may also need adjusting, the 10k potentiometer may not have enough adjustment for your needs.

So there you have it – the schematic for a sensitive 60kHz receiver. Feed the output of this into part one of the schematic and you’ll have a perfectly working, completely accurate clock!

The circuit could be improved significantly by ditching the germanium rectifier diode in favour of a system that does not give us a voltage loss. There are plenty of example circuits around for op amp based rectification circuits, but I didn’t have the veroboard space to try some other rectification methods.

PSU / decoder: schematic part one

Part One I’ve just moved back to University for my final year, and as such have found some time to draw up half of the schematic for the rugbyPICclock! I was pleased to find that as soon as I arrived in Bath, I plugged in the clock, it received the signal fine and was displaying the time in no time.

Shown above (click for a big version) is the schematic for the digital part of the circuit. This schematic has four inputs, Mains Live, Neutral and Earth and the digital signal input from the receiver. A common cathode 6 digit LED display is connected to the MAX7219 in the schematic. Exact details on how to go about connecting the display up to the MAX7219 are given in the datasheet.

Note that if you are building this circuit you should be very careful when dealing with the mains parts if you don’t have experience, you could really seriously injure yourself. As an alternative to the power supply, you can buy a 12v DC power supply (or use a 12v battery) and connect it to the ground and input pins of the 7805. You should bear in mind though that the receiver won’t work as well if the 0v isn’t connected to mains earth. Thinking about it, connecting the mains earth to the low voltage parts of the circuit probably goes against some kind of wiring regulation. The general rule is fairly obvious, just don’t touch mains parts in the PSU when it’s on. That way you’ll live. 🙂

I tried for ages to find a decent free program to draw the schematic in a pretty fashion, but couldn’t find one so in the end resorted to drawing it (messily!) by hand.

I’ll post up the PIC code soon (or can email it to you if you mail me), because it’s still a work in progress. Losing my PIC Programmer power supply in the Cheltenham to Bath move doesn’t help.

Just as an aside, I’ve now confirmed that the clock receives a signal and works fine in Cheltenham, Bath and Stevenage, all in the south of England.

I’ll get to work on the receiver and demodulator schematic. This is nowhere near as simple as this schematic though, so it may take a while.