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PC or PICAXE interface for UHF remote control For those wishing to experiment with home automation or extensive remote control, the popular PICAXE chips or a personal computer can be interfaced with the 4-channel transmitters and receivers from Oatley Electronics, as featured in the June 2003 issue. By JOHN HOLLIDAY T HE ORIGINAL Oatley Electronics 4-channel UHF transmitter board had manual pushbuttons but these can be omitted for computer control. In fact, one transmitter can control the four channels on up to 16 separate remote receivers – that’s up to 64 computer channels altogether! This circuit concept uses the eight data lines from a PICAXE18A or the parallel printer port of any PC. Since the PC situation is the more complex, I will limit the article to describing how to interface the parallel printer port of a PC to the transmitter. I will assume the reader is familiar with the article in the June 2003 issue. The circuit The circuit uses a quad NAND Schmitt trigger gate (74HC132) and four PNP transistors (Q2-Q5) to replace the four pushbuttons in the original design. All lines from the computer’s parallel port are connected via 470Ω resistors and all have 22kΩ pull-down resistors, to avoid damage to the NAND package or to IC1. Fig.1 shows how the four lowest data lines (lower nibble) are used to control the transmitter encoding pins (1-4) of IC1. So instead of these pins being permanently hard wired into a fixed encoding pattern, they are now under computer control. The remaining encoding pins of IC1, pins 5-8, are left unconnected. Because encoding pins 1-4 can be either high or low, as determined by D0-D3, up to 16 different combinations are possible, so that the transmitter can individually control up to 16 different receivers, each with its own unique code. Note that because the data lines can only be high or low, the “open circuit” condition of the transmitter security encoding cannot be used in this circuit. This does result in less combinations being available but that does not reduce the utility of the circuit. Data on D0-D3 represents some binary number, so if we output the number 10 (1010 binary) using these four data lines, this effectively encodes transmitter pins 1-4 as LOW, HIGH, Table 1: QBASIC Code lowernibble = 12    'set variable “lowernibble” to required receiver code channels = 14    'set variable “channels” to required channel combination uppernibble = 16 * channels    'put channel combination into the upper nibble outputdata = lowernibble + uppernibble 'combine the two nibbles OUT 888,outputdata    'put the data on the data lines D0-D7 (LPT1) Once you know what you are doing, the above lines can be condensed into one: OUT 888,(12 +16*14) www.siliconchip.com.au The Oatley Electronics 4-channel UHF transmitter board includes an SM5023 trinary encoder IC and a pre-built UHF transmitter module (mounted on the underside of the board). LOW, HIGH. Therefore a receiver with its corresponding decoding pins 1-4 hard-wired in the same pattern (LOW, HIGH, LOW, HIGH) would accept the transmission of data while other receivers with different hard-wired patterns would not. From here on, I will identify a receiver by the binary number (or decimal equivalent) of the hard-wired pattern of pins 1-4 on its decoding chip, with pin 1 being the least significant bit. As with the transmitter, pins 5-8 of the decoding chip are left unconnected. As noted above, the four pushbutton switches on the transmitter (IC1) in the original design have been replaced by four BC558 transistors. Each transistor and its associated 10kΩ base resistor can be conveniently installed in the four holes vacated by the pushbutton switch. Each transistor is driven by a NAND gate (74HC132) which in turn is controlled by a data line from the printer port. The four channels of the transmitter are thus controlled by the upper four data lines (upper nibble) D4-D7. If D4 is low, for example, the output pin 3 on the 74HC132 is high and Q2 is therefore off. This corresponds to channel A being off. If, on the other hand, D4 is high, Q2 will be on, thus turning channel A on. A similar situation holds for the other three channels April 2004  75 Fig.1: the interface circuit uses a quad NAND Schmitt trigger (74HC132) and four PNP transistors (Q2-Q5) to replace the four pushbuttons in the original design. – they will be on if their data line is high and off if their data line is low. In a similar manner to the encoding pins, the four channels can be turned on in any combination by using the appropriate binary number output on data lines D4-D7. If, say, we output the number 7 (0111 binary) to the upper nibble, channels A, B and C would be on, while channel D would be off. As each individual channel is activated by its own data line, each channel can be thought of as the number represented by that data line. So channel A alone is turned on by outputting “1” (0001), B alone by “2” (0010), C alone by “4” (0100) and D alone by “8” (1000). That means that the channels 76  Silicon Chip are individually represented by the successive powers of 2. If we wanted to turn on both B and C but no other, we would output 6 (2 + 4 = 0110). If we wanted all channels on we would output 15 (1+2+4+8 = 1111), while outputting 0 would turn all channels off. That is, to turn on a combination of channels, we simply add the numbers representing the individual channels. Software Before exploring this issue, we might now tackle the software problem of putting the correct data on the correct data line. I will illustrate this using QBASIC commands. First, de- cide which receiver is to be addressed; let us suppose it is the one coded 12. This means the lower nibble needs to be 12. Next, decide which channels on receiver 12 need to be turned on; channels B, C and D, say. This means the upper nibble needs to be 14. The printer port address is 888 (decimal). The QBASIC code shown in Table 1 could be used. One last word of warning. If you write some software to control items in the home, you need to think about the consequences of what might happen if some piece of equipment is turned on when it is not supposed to be on. How can this happen? Even if your software is perfect, never allowing such terrible www.siliconchip.com.au things to happen while the software is in control, the computer is not always under software control. During the boot up process (or an automatic reboot after a power supply failure), your computer (including the printer ports) is at the mercy of the BIOS, MSDOS or perhaps Windows. The amount of fiddling that can go on with the data lines of the printer ports during the boot up process is beyond belief. With my computer, when booting with MSDOS 6, no data lines on port 888 were set high, while with Windows 98, D3 was left high. This means that the computer is quite safe to boot using a QBASIC control program running under MSDOS. It is also safe to operate under Windows 98, because while D3 is high and could be a valid www.siliconchip.com.au receiver address, the upper nibble data lines are always low, meaning no data will be sent. This was the reason for assigning the lower and upper nibbles the way they were. You will have to monitor the data lines of your own system to see if there might be a safety problem during the boot up process. This is where the 74HC132 might come in handy again. Instead of making pins 1, 4, 9 & 12 permanently HIGH, they could instead be connected to a control output which could be kept LOW during the boot up process. The control output would then be changed to HIGH under software control. Suitable control sources might be found in the auxiliary output channel 890 or by using one of the data lines D0-D7 which remain unaffected dur- ing the boot up process. As for using the PICAXE as a control source, the 18A is ideal, having eight dedicated output data lines. The 08 suffers from only having four output pins. Here we could use two outputs for addressing and two switching channels. This would allow four receivers to be addressed but only two channels on each could be switched. The available outputs could also be split in other combinations according to your need. So go ahead, the whole world of home automation lies before you. SC Footnote: further information on parallel port interfacing and programming is available from www.lvr.com and www.beyondlogic.org April 2004  77