This article will show you how to build your own version of the Fire-Stick infrared remote control system. The Fire-Stick has been an extremely popular, and HOT selling item here at Rentron.com for quite a long time. The LITEON infrared receiver modules originally designed-in to the Fire-Stick have been discontinued, and forced us to re-design the original circuit boards.
Since we're completely re-designing the Fire-Stick, we decided to share the original design with our visitors, and show you how to squeeze some pretty incredible operating distance out of an infrared remote control system.
Here's the transmitter schematic.
How it works:
The heart of the Fire-Stick is the Holtek HT-12A encoder IC. This handy little encoder was specifically designed for infrared transmission media, and internally generates the 38KHz carrier frequency for each data transmission.
The data pins of the HT-12A act as both data input pins & transmit enable inputs. Unlike most ordinary encoder ICs with separate transmit enable pins, and data input pins, the HT-12A uses the data input pins to take care of both input functions.
By pulling one of the data pins to ground, the transmission cycle is started. As with most encoders, a pre-determined sequence of events takes place during each separate transmission cycle. The HT-12A however, has a few internal extras that make it pretty flexible. Notice from the flow-chart below that the logic state of the L/MB pin is checked prior to finishing the transmission cycle and returning to standby mode.
To L/MB or not to L/MB:
The L/MB pin serves as the Latch/Momentary selection pin, and determines whether the HT-12A will end each transmission cycle by sending the exact logic state present on each data pin, or end the transmission cycle by sending logic "1" 7 times as the data. If L/MB is left floating, ( not connected ), the transmission will send only the actual logic values present on each of the HT-12A data input pins. This logic value will be latched onto the output pins of the HT-12D decoder IC until you send another transmission with different data.
The HT-12D decoder IC used on the Fire-Stick receiver normally operates in latched mode. Without the added feature of the L/MB function of the HT-12A encoder IC, the receiver would normally operate in latched mode. Operation of the transmitter causes corresponding data pins of the HT-12D decoder IC to latch at logic 0, or logic 1, depending on the actual logic values present on the HT-12A data input pins, and the state of the L/MB pin.
Using the L/MB option of the HT-12A to select momentary operation of the HT-12D will allow the receiver output pins to be momentarily pulsed low, and then return to logic 1 once the transmission ends, and you release the HT-12A data input pin from ground.
Note: To begin a transmission using the HT-12A, one of its data input pins MUST be connected to ground, hence at least one data input pin of the HT-12A is logic 0. Any combination of data input pins may be connected to ground to form specific bit-patterns to be transferred to the receivers decoder IC. The output of the receivers decoder IC will reflect the physical logic state of each data pin on the transmitter, providing the following conditions are met.
Example: You connect D0 on the HT-12A to ground. A transmission cycle is initiated and data output pin D0 on the receivers decoder IC will now be pulled to ground. The remaining data output pins on the receivers decoder IC will be a logic 1.
Any data input pin on the HT-12A not connected to ground will default to logic 1. Logic 1 is the default logic state for all data input pins of the HT-12A not physically connected to ground because of the internal pull-up resistors inside the HT-12A..
These internal pull-up resistors cause each data pin not connected to ground to be transmitted as logic 1.
Who needs an address...?
The logic state of all address pins on both the encoder & decoder ICs must be 100% identical or no change will be seen on the decoders output pins. The address pins allow for a certain degree of security, and are beneficial if a similar transmitter will be operated within range of your receiver. This simple method also provides an efficient means of controlling multiple receivers in close proximity to one another with a single transmitter.
With different addresses on multiple receivers you need only adjust the address of the transmitter to match that of each receiver to gain control of that receiver. With 8 address pins, A0 - A7, a total of 256 possible address combinations are possible. This means you can control up to 256 separate receivers with a single transmitter, or have up to 256 possible security addresses for a transmitter/receiver combination.
Here's what an actual transmission cycle looks like:
The synchronization period ensures the receiver decoder is synchronized with the incoming serial data bit-stream. The decoder IC knows that immediately after the detected 1/3 bit synch period, data for the address will be arriving. During reception of the address, the decoder IC validates each address-bit by comparing each bit to its own local address.
If the received address does not match, the remainder of the transmission will simply be ignored, and no logic change will be seen on the decoder outputs, nor the VT pin which signifies a valid transmission has been received. An address match will cause the remaining incoming data to be transferred to the decoder data output pins, and the VT valid transmission reception pin will toggle from logic 0 to logic 1.
The decoder ICs data output pins will remain at their last logic state (latched) until another valid reception is received, and data has changed, or the L/MB function pin had been selected to initiate momentary operation. In which case the data output pins will operate momentarily.
Note: The VT pin will only remain at logic 1 during reception of a valid transmission. Ending a transmission -- or any transmission not pre-validated by a matching address -- will return the VT pin back to its original default state of logic 0.
The VT output pin can be used to generate an interrupt condition in processor-based systems, and cause vectored jumps to interrupt handling sub-routines to further process the incoming digital data.
Tip #1: VT can also serve as a control output to connect or disconnect power to the receiver section control electronics to conserve power during non-valid transmission reception periods. This ability can be a very helpful feature in low-power or battery based systems, and can help extend the operational life of the receiver power supply system considerably.
Tip #2: The HT-12D decoder IC will return to stand-by mode and require as little as 0.1 to 1uA stand-by current as long as there is no logic high signal applied to the DIN pin. If the receiver will operate in a noisy environment, IE,, enough stray EMF to generate sufficient signal to cause the DIN pin to see fluctuating logic signals on DIN -- you can eliminate this condition with the addition of a single pull-down resistor.
Connecting the DIN pin to circuit ground through the pull-down resistor will hold the DIN pin at logic low, and ensure the HT-12D returns to sand-by mode during periods when there are no transmissions. Choose a resistor value of approximately 10K or more. This will apply sufficient pull-down to eliminate weaker logic high signals injected into the circuit from stray EMF.
A signal generated from a noise source would have to be of sufficient magnitude to overcome the logic signal applied to the DIN pin through this pull-down resistor, and is highly unlikely.
Tip #3: How can I tell if there's sufficient noise in the area to cause false triggering..? A simple method it to attach an oscilloscope to the DIN pin with the scope set to view AC. If you see an AC voltage element on the DIN pin -- use the resistor. In most instances, you'll see an AC element present that's produced by other equipment around you. Normally this isn't of sufficient magnitude to actually cause false triggering of the DIN pin, but an ounce of prevention is worth a pound of cure, and the addition of the resistor is of minimal cost. Much cheaper than batteries these days at any rate...!
Click HERE For Part-2
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