Nuts and Volts - March 2015

Unfortunately, the FTDI cable has a downside. Even though it’s cheaper than the AXE027, it’s still relatively expensive — especially if you want to work on more than one project at the same time.

I originally chose to use the FTDI cable because it was the only cable that I could find that included the capability of inverting the polarity of the TxD and RxD signals in software, which is necessary for the PICAXE programming interface. As far as I know, that’s still the case.

However, this month, we’re going to explore a hardware solution to inverting the TxD and RxD signals so that a wide variety of relatively inexpensive USB-to-serial adapter cables can be used for our PICAXE breadboard projects.

In the process of writing this month’s column, I experimented with a couple of different logic gates (inverters and NAND gates). A logic gate can certainly be used to invert a data signal, but it turns out that the most space-efficient solution (at least for non-SMD parts) is to simply use an NPN bipolar transistor for this purpose. I chose the 2N3904 NPN, but any general-purpose NPN transistor should do.

Experiment 1: Using an NPN Transistor as a Data Inverter

Figure 1 shows the schematic of a simple NPN data inverter. The schematic doesn’t show the standard PICAXE programming circuit, but, of course, you need to include one — just use whatever programming connection you prefer.

The circuit in Figure 1 may look familiar. That’s because we’ve already used an NPN transistor as a switch back in our first LED-multiplexing project way back in the December 2009 Nuts & Volts. Since that’s quite some time ago, let’s quickly review the basics of transistor switches.

First, as you may remember, the three junctions of a bipolar transistor are referred to as the base, the collector, and the emitter. (In Figure 1, they are labeled “B,” “C,” and “E,” respectively.) Second, the standard symbol for an NPN transistor includes a small triangle pointing away from the base at the emitter connection.

Third, when using an NPN (or PNP) transistor as a switch, the general rule is that the switch will turn on (i.e., current will flow from the collector to the emitter) whenever the base is “taken toward” the collector, i.e., whenever the base is connected to the same voltage level as the collector.

Finally, the transistor is not a “perfect” switch. In other words, there’s a small internal voltage drop between the collector and the emitter whenever the transistor switch is “on.” For my setup, I measured the drop to be about 0.3V.

In order to understand how the transistor switch in Figure 1 functions

USB-to-Serial Adapters Revisited

Back in the June 2010

installment of the Primer, we discussed the FTDI USB-to-TTL serial cable, and how a single cable can be used to both power and program a PICAXE breadboard circuit. The FTDI cable is simple to use, plus it consumes much less breadboard space than the standard AXE027 programming cable and a separate power supply. As a result, I almost always choose the FTDI cable whenever I'm setting up a new PICAXE breadboard project.