Soldered version of the PO-32 to Arturia MicroBrute sync

 As I still had a few prototype PCB boards I decided to make a soldered version of the syncer I previously designed. I added a power plug to it, so I can splice the power off the MicroBrute into this circuit. The power circuit is the usual LM7805 implementation seen elsewhere on this blog. As you can see from the pictures, my soldering skills haven't improved, but they suit the purpose, which is to make a nearly unbreakable version of the board.

I like my trick of putting the connectors at the edge of the board like this, so I don't need to drill larger holes for the pins. I chose pink for the connector to the PO-32 because Teenage Engineering sounds more playful, and because that plug is red, so I know what to match it to. Next will be to set this up and start making a song with it.

There was one soldering mistake that I had to fix, a blob of solder had fallen between two of the lines. This caused the circuit to just play a single note initially. But with my multimeter it was easy to figure out.

Phone charger for bicycle

The latest project I made was the USB port for my bicycle. I had the circuit ready a while back, but I hadn't moved my Nexus in-hub alternator to my new bike yet. The Google bike to work day made that easy, as they had a bicycle workshop where they were quite helpful and making the change (I couldn't do it myself due to lack of a good tool to undo the old wheel: it had been tightened a lot).

Initially my circuit had only a 10uF capacitor after the diode bridge, this turned out not to work and confuse the phone a lot. After increasing this to 230uF by adding a 220uF capacitor everything worked well, but a 470uF probably works even better.

Here you can see the circuit actually attached to the bike (using electrical tape for insulation). You can see the Nexus wheel at the bottom and the USB connector with the plug heading to the phone. As you can see there is one wire hanging loose, this is the ground wire, which is not needed as the alternator itself is already grounded to the bicycle. I prefer to leave it disconnected to avoid ground loops, although it is unlikely this will happen.

There is also a switch, shown here from the side, which allows you to turn off the circuit (which will make it easier to cycle, as there is hardly any resistance from the in-hub alternator if there is no draw) or switch the power directly to the front light of the bicycle, which was the original configuration.

The final question is whether it works, and it does. The first trip I got 2% charge (from 28% to 30%) the second trip 5% (from 90% to 95%) and the third trip 8% (from 66% to 74%). This is while the phone is operational (and actually syncing Email in the Mountain View WiFi area).

Current project: Drake-TV

My current project is Drake-TV, a new version of the Drake that is hooked up to a PS/2 keyboard and the television, and works on a 9V adapter. It contains the various circuits described on these pages: the power supply (using a 7805), the video output (this time using a 16F688 instead of the 16F628A), the main processor (the 16F690) and the "external" memory EEPROM (24LC512). The audio has not been connected yet, but will undoubtedly be connected in the future.

Serial port for the new GPS

Version 2 of the GPS has a more complicated serial port interface, as it has two functions. The original only had to read from the GPS, and store the results in memory, the new version also has to be able to send the result to a PC. Unlike version 1, which used an external GPS and therefore used a regular serial input, version 2 uses an actual GPS module. The second version also has to be able to output to a PC, something that was not covered at all in the first version.

The connection to the GPS module has some intricacies, because the GPS module, despite having a 4-6V input, had only a 3.3V output. Glancing through the data sheets of the PIC16F628A this might seem to be no problem, until one notices that the serial input port of this microcontroller uses Schmitt Trigger inputs, which means the minimum voltage for a '1' is 4V. To do the conversion I used a 74HCT125, which accepts a large range of input voltages, and converts them to whatever voltage is set on Vcc. Of course it is a bit of a waste of the other 3 buffers inside this chip, but it works and makes it very unlikely to damage the GPS module. An alternative would be to run the PIC16F628A on 3.3V, but this would cause other difficulties, for example with the interface to the LCD display, which needs 5V. Apart from the buffer, the connection to the GPS is very simple, as there is no other conversion necessary.

To connect to the PC we would officially have to output +12V and -12V signals. Clearly this is not easy with 5V power, and even if we used the 9V battery power we would still run into trouble with negative voltages. Fortunately there is the DS275. This small component leeches voltage from the RS232 interface of the PC and uses that voltage to send the messages to the PC. Of course, a PC might not have the right voltage (in fact, often they do not) but you would expect a PC to be able to read its own voltage levels back. The DS275 covers the -12V, for the +12V it uses Vdrv, which in this case is still connected to +5V. As the RS232 specification states that anything above +3V should be considered legal, +5V is fine, though barely. Hooking up the DS275 is very staightforward. As you can see it handles reads as well, but in this case it is not used. However, on the PC side the read should still be connected to allow access to the voltage levels. Note that if you decide to use the DS275 for sending AND receiving you should realise the component is NOT full duplex. You can either send or receive, but not both at the same time. If you need to do this, you might want to consider the Maxim 232 range of devices, which will be covered at another time.

Of course, you can see the decoupling capacitors of 0.1uF everywhere in the schematic. The GPS, the DS275 and the 74HCT125 all need them. Also, the plug used for the serial port is female, in this case, and this is why pins 2 and 3 were swapped compared to the previous schematic.

More power (on voltage doubling)

The calculator uses 2 AAA batteries. However, so far all my circuits use 5V, and even the HD44780 LCD display requires 5V. Initially I decided to use 4 rechargeable AAA batteries (1.25V each, 5V total) but this was too cumbersome and heavy.

I had already used the ICL7660A to invert a voltage. This time I decided to use it to double a voltage. The circuit above comes straight from the documentation, and it works. However, as you can see, there is a diode involved, which causes the voltage to drop by 0.7V. So the 3V becomes 5.3V. Clearly there is a risk involved here: if the batteries are exceedingly fresh, they will deliver over 1.5V each and the 5.5V threshold of the PIC microcontroller could be reached. However, this has not happened yet. Also, rechargeable batteries don't work well in this configuration, because they have too little voltage to properly power the LCD display.

In the schematic the +5V is not 5V that is applied, but actually the voltage that comes out of the circuit. The battery is hooked up to the 3V input. Despite the odd configuration of the capacitors, they are correct.

A more ideal way to create the 5V is to use a bucket converter, but that will not be covered in this post.

Power

All designs require power. Here I cover the simple design that I use for most power supplies. As you can see from the schematic, most of the work is done by a component that is called the 7805. The 7805 model is the biggest version, and recognizable by its large heat sink, which has a hole so it can be attached to an even bigger heat sink. The 7805 model can output 5V at 1A, which is quite a lot and suitable for projects involving motors, like my servo controller. A smaller model, the 78M05 gives 500mA, and is therefore not very useful. I include it more as a warning: avoid the 78M05. The smallest model is the 78L05, which looks more like a simple transistor. The 78x05 series is inexpensive in terms of money, but not entirely cheap in terms of power consumption. They regulate power quite well, ensuring that their output is 5V (the 05 in their name is this voltage, a 7812 would output 12V) with very small fluctuations. They do require that the input voltage is higher than their output voltage, though, and the difference is converted into heat (hence the heat sink). For 5V output the input needs to be at least 6.6V (or higher, check the data sheet of the particular model you acquired). There are other options, and I will discuss some later in other posts to this blog.

Note that the component does not limit the current to the value specified. All it does is try to maintain the output voltage. I have had a 78L05 output over 1A at 5V. The problem here is mostly the heat: exceeding the output current will cause the component to grow hot, to the point where it will burn you, or even destroy itself. It is therefore wise to know what the current consumption of your circuit will be, and choose the appropriate component.

As you can see from the schematic, there are two other components, both of them capacitors. At the input side I have a 10uF capacitor. This is to ensure that fluctuations in the input voltage are filtered out, as well as moments where the target circuit has peaks in the demand. It generally does not hurt to increase this capacitor: values up to 470uF are quite acceptable. Note that this capacitor has polarization and will therefore need to be inserted correctly. On the output side I usually suffice with a 0.1uF capacitor. Obviously it is possible to add a larger capacitor on this side too, but the ceramic capacitors are much faster than the electrolytic ones, and response time is generally more important. Apart from the one in the power circuit I also put the 0.1uF capacitors near all the current consuming components on the board. This is called "decoupling" and without them you might experience very strange behavior, as voltages might drop below operating requirements. If you feel that ceramic capacitors are not fast enough you could switch them for tantalum. However, tantalum capacitors have polarity, which is why I tend to avoid them.

In the circuit above I assume that it is hooked up to the mains using a wallwart. However, frequently I use batteries. As you need to exceed 6.6V my preferred method is a 9V battery. For high current applications this has to be a NiCd battery, as these give a lot more current than other batteries. This might seem odd, given that a 9V NiCd battery generally has "160mAh" written on it, which implies a small amount of current compared to a NiMH battery (300mAh). However, the C (charge/discharge rate) of a NiCd battery is 5C-10C, so it can actually provide 800mA-1600mA, whereas a NiMH usually doesn't exceed 0.5C, which means it would only provide 150mA. Of course, the NiCd battery only provides this power for 6-12 minutes, whereas the NiMH would last 2 hours. I'm not a fan of batteries that can not be recharged, because I sometimes forget to disconnect them which renders them useless, and because they are not good for the environment and difficult to dispose of.