Circuit Schematic and Project Holtek HT12E encoder in high-density ASK R-C applications using 74HC221

Radio. - Circuit Schematic and Project Holtek HT12E encoder in high-density ASK R-C applications using 74HC221. The high-density ASK (amplitude shift keying) environment is one in which there are so many transmissions occurring at a given frequency that they occasionally collide and thus fail to get properly decoded by the targeted receivers. There are a number of potential solutions, but in this discussion we are concentrating on limiting the transmitter On-period. This is accomplished by operating the encoder IC at the maximum data rate and by limiting the transmitter On-time via a monostable multivibrator. Obviously, this is not appropriate for critical applications, but provides an inexpensive solution for many non-critical applications. For a complete mini-system, the HT12E encoder must be matched with a HT12D decoder.

Battery life

Battery life should be excellent as both the monostable and encoder are CMOS –neither draw measurable current when at rest. Limiting the transmission time also reduces battery load. As a result, the battery may never need replacement. The way the 74HC221 monostable works is that there is no voltage across the timing resistors when the device is at rest –without voltage, they cannot add load. Note that not all monostables work this way –some have significant standby current.

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