General purpose display decoder
Designing a suitable LED display particular application can be time-consuming. In particular, the decoder that sits between the driver circuit and the display often needs to be made to measure. The main advantage of the circuit presented here is that the same hardware can be used to implement many different types of display decoding. This is achieved by using an EPROM, which most readers will be able to program or have programmed.
The circuit translates a 12-bit input code into suitable drive signals for a 4-digit readout built from 7-segment common-anode LED displays. Four digits (displays) require four bytes in the EPROM. If these bytes are stored at four successive addresses, they can be read by applying binary code 00 through 1 1 to the two least significant address lines, AO and A 1. Since the four displays are multiplexed, driving the AO and A1 lines in this way enables codes to be fetched from the EPROM and indicated on the readout. This leaves the remaining address bits of the EPROM free to apply the code to be converted.
As an example, an EPROM is programmed to function as a code translator for an RC5 infra-red remote control transmitter. The RC5 receiver described in Ref. 1 may be connected directly to the decoder input connector. K1 via a short flat cable. An RC5 code consists of five address bits and a 6-bit key code. In this example, the two most significant digits are used to show the address, and the other two to show the key code. The six data bits. the five address bits and the toggle bit are connected to the EPROM address lines. The toggle bit is used to drive the decimal point on the digit at the extreme right. which provides an indication that a key has been pressed on the remote control unit.
The contents of the EPROM are generated with the aid of the Pascal program shown. When analyzing the program, bear in mind that the Outputs of the IR receiver are active low. This means that counting has to be done backward, that is, from 31 to 0 for the addresses and 63 to 0 for the data, rather than in rising order, 0 to 31 and 0 to 63, respectively. In this way, many decoder variants may be implemented simply by adapt-the EPROM contents as required. The EPROM to implement the RC5 code readout is available ready-programmed from Elektor Electronics (Publishing).
Universal RC5 code infra-red receiver, Elektor Electronics January 1992.
R1-R8 = 220 Ω
R9-R12, R14 = 10 kΩ
R13 = 100 kΩ
R15, R16 = 6-stage 100 KΩ SIL
C1 = 10 nF
C2 = 220 μF, 16 V
C3, C4 = 100 nF
T1-T4 = BC557B
IC1 = ULN2804
IC2 = 74FICF139
1C3 = EPROM1 Type 27128 Ref. 6261*
IC4 = 4060
K1 = 20-way straight box header
LD1—LD4 = HD11050 (Siemens)
Enclosure, e.g. Heddic Type 222
PCB Ref. 934029*
• available from Elektor Electronics (Publishing)
Representative in UK: ElectroValue,
3 Central Trading Estate, Staines
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