Tuesday, April 5, 2016

Circuit Schematic Holtek HT12E encoder in high-density ASK R-C applications

Eltronicschool. - This is one of Radio circuit schematic categorize that titled with Circuit Schematic Holtek HT12E encoder in high-density ASK R-C applications like in Figure 1. below. 

Circuit schematic

Component Parts
  1. HC221 IC
  2. Holtek HT12E IC
  3. Resistors
  4. Capacitors
  5. All components see in Figure 1 above
Description

Circuit schematic like in Figure 1 above is Circuit Schematic Holtek HT12E encoder in high-density ASK R-C applications. According Electroschematics site describe that he 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.

  • Key contact de-bounce multivibrator

The most significant problem occurs when the operator keys the encoder for an extended period of time. This essentially jams the frequency so that no other transmissions can be received. To prevent this from occurring, a monostable multivibrator keys the encoder for about 20mS. This limits the transmission to the minimum 4word data burst. To prevent the transmitter from being keyed by rapidly pressing the key button, another section of the monostable multivibrator prevents successive keying until a 1.2sec timeout period has elapsed.

The 74HC221 non-retriggerable monostable multivibrator is ideal for this function. Do not confuse this device with the 74HC123 retriggerable monostable multivibrator that has the same pin-out.

  • 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. Read more here.


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