Friday, 27 November 2015

Building a Custom Integrated Circuit Package





Whilst working on the update for the μVolume project, one of the features I wanted to test was the ability to not only receive infra-red signals from a remote control but, to also transmit emulated infra- red signals. This could be used to control such a device as a TV or surround sound amplifier. An IR transceiver package was the obvious choice! After a lot of searching I concluded that the available parts were all basically costly, tiny SMD packages intended for the transmission of data between devices within a 1 metre range, such as laptops and mobile devices.

These additional components could be included on the actual PCB, however, that would have involved a re-spin. Plus, I prefer to keep the current PCB tidy and free of floating bodge wires and components.

Each Atmel 32u4 pin has a current rating of 20mA which would not give a usable range, so a transistor was incorporated to deal extra current from the 100mA IR led.

The Vishay TSOP4838 and similar IR receivers come complete with all the necessary components including an amplifier section, plus IR led giving a maximum range of 45 metres all built into a nice three pin part.



The aim of this pictorial guide is to build a complementary IC package to the Vishay IR receiver. It incorporates the same pitch and pin out except for the "Output(1)" which will now become the new input.

 Colour Coding the Components:


It is helpful to start the thinking process with a schematic of the circuit, hand-drawn or otherwise. I found it more intuitive to colour code the mating legs to avoid mistakes.


Quick Fact:

TO-92 descriptor is derived from the original full name for the package: Transistor Outline Package, Case Style 92. (plastic encapsulated package with three leads)

The B.O.M (bill of materials) for the construction of the TO-92 style package for the IR transmitter are as follows...

  1. 2n2222 NPN Transistor
  2. 220ohm Resistor 
  3. IR 940nm LED (940nm gives the best infra red frequency range for remote control applications)



Construction:


After what we call in Yorkshire "A good coat of looking" I was ready to proceed...


The two largest components, the 2n2222 transistor and IR LED, are bonded together with epoxy thus, making a stable platform for bending of the leads to the other mating surfaces.


Forming the leads:

When bending the component tails, thinking one move ahead is crucial.
The marked greens, Transistor collector(3) & LED cathode, tails are folded on top of one another, then soldered, not soded ;)


The transistor "Emitter(1)" is bent to a virtual middle. This forms a new ground "GND(2)" in the middle of the finished package.
The most logical position for the transistor "Base(2)" was to fold it upwards.


In simple terms, the resistor gives some protection to the MCU or whatever is connected, whilst also protecting the transistor by limiting its base current.
The resistor can now be soldered and positioned down the side on its return to form the finished tail, which is now known as "Input(1)".


The transistor "Collector(3)" is left in it's original position but is bent slightly outwards to form the last tail on the finished package known as "VCC(3)"



Solder is applied to form a gusset to mechanically strengthen the corner. Using a similar pitch header the lead alignment was checked before encapsulation.


Encapsulation:

A small strip of tape, in this case Kapton, is used to protect the IR LED lens against contamination from epoxy during encapsulation.

The method of encapsulation uses a cheap syringe based two-part epoxy and a tint such as powder paint or toner.  I have previously talked about the process here...


The New Pinout:

Now we have a complementary pinout to the TSOP4838 IR receiver I discussed at the start...






Reminds me of a cartoon character somehow, ideas?


Notes:

Some may wonder why there is no series resistor on the LED. The signal to the LED is not constant but pulsed. Whilst it may be good practice, I deemed it unnecessary.


Simple IR Transmitter Circuit principals:


The micro processor, connected to input (pin 1), generates a high/low pulse (IR remote signal zeros & ones) at a low current to the transistors "base" (think relay).

The emitter then sources the available higher current from its source (+5v) then, pulls it down to the collector (the LED cathode which is GND).

The LED anode is tied to (pin 3) VCC +5v thus, the LED now lights in time to the pulses at its full power (100mA).







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