The modern Polaroid SX-701 camera (1972) became once a shock of engineering: the realm’s first instantaneous SLR camera.
This iconic camera became once the brainchild of Dr. Edwin Land, a genius who co-founded Polaroid, invented polarized sun shades,
helped manufacture the optics for the U-2 peek airplane, and created a thought of coloration imaginative and prescient.
The camera ragged self-atmosphere up movie2 with
square photos that got here into scrutinize over a couple of minutes.3
The movie became once a advanced sandwich of 11 layers of chemicals to build a unfavorable image after which make the viewed coloration image.
However the movie became once merely one of the necessary camera’s enhancements.
Edwin Land and the Polaroid SX-70 camera were featured on the quilt of Lifestyles magazine, October 27, 1972.
The camera required advanced novel optics to enhance the intricate light direction confirmed below.
The parts included a flat Fresnel judge, aspherical lenses, and a transferring judge.
These optics would maybe well focal level from infinity the total contrivance down to a closeup of 10 inches.
The optics are even more unbelievable if you happen to think that the camera folded flat, 3 cm thick and in a obtain to slot in a jacket pocket.
Map from Lifestyles magazine, Oct 1972, showing the light direction by contrivance of the camera.
However I will focal level on the camera’s electronics, powered by a personalized flat battery pack.
When the shutter button is pressed, the camera conducted several initiatives with precision timing.
First, a solenoid closes the shutter, blocking off the entry of sunshine into the camera.
Next, the motor is grew to change into on, causing the camera’s inner judge to flip as a lot as uncover the movie.
The solenoid is then de-energized, causing the shutter to open.
The movie exposure time is dependent on the light stage; at the kindly time, the solenoid closes the shutter all but again to pause
Ultimately, the motor runs all but again to eject the movie and reset the judge.
I’m serving to the openSX70 project by reverse-engineering the chips on the exposure lend a hand an eye on board.
This board contains three surface-mount ICs: a chip to be taught the light intensity from a photodiode,
a timer chip to lend a hand an eye on how long the shutter blades are open, and
an influence lend a hand an eye on IC to force the motor and solenoids.4
The exposure lend a hand an eye on circuit board, manufactured by Texas Instruments. Characterize from openSX70.
The trend of this board became once contentious, with
Fairchild and Texas Instruments battling to
offer the electronics for hundreds of hundreds of cameras.5
The exposure lend a hand an eye on board went by contrivance of three designs as Fairchild and Texas Instruments struggled to satisfy Polaroid’s
tag target of merely $5.75.
First, Texas Instruments built a lend a hand an eye on board from a ceramic substrate with laser-trimmed resistors.
The costly parts attach the board advance over finances at $100 so Polaroid ragged these boards only in prototype cameras.
Fairchild’s manufacture became once popular by Polaroid even when its tag of $20 composed exceeded the target.
Fairchild’s board became once ragged from 1972 to 1973, nonetheless Texas Instruments fought abet with an all-novel manufacture that tag only $4.10.
This TI board became once the long-term winner, and is the one I am examining on this publish.
The optical chip
The exposure lend a hand an eye on board mechanically adjusts the exposure time based entirely totally on the amount of ambient light.
Ambient light is measured by the optical chip, a bundle that combines a photodiode and a silicon die in one limited bundle.
The silicon die is trusty by epoxy, nonetheless the increased photodiode is uncovered so external light can tumble on it.
The optical chip contains a photodiode and a silicon die in one bundle.
To measure the ambient light, the chip implements the integrator circuit below.
The photodiode generates a limited fresh that is dependent on the light stage.
This fresh is built-in over time using a capacitor unless a threshold is reached.
By opening the shutter at some level of this interval, the movie is uncovered for the specified quantity of time.
(The movie’s exposure is dependent on the overall quantity of sunshine received, which is the an identical tag that the mixing calculates.)
The op-amp die outputs the voltage at some level of the capacitor with out draining the capacitor at some level of.
The optical chip is in fact an integrator.
The photo below reveals the silicon die below a microscope.
The chip, made by Texas Instruments, is dominated by the zig-zags forming two interlocking JFET transistors.
A JFET is a particular form of transistor, ragged sooner than MOSFETs grew to change into standard.
These transistors dangle very low input currents, so that they would maybe additionally no longer drain the capacitor as it expenses.
The interlocking structure ensures that each transistors are at the an identical temperature, so the circuit will end staunch even when
the chip heats up inconsistently.
The chip additionally contains NPN and PNP transistors, resistors, and a capacitor (the kindly pink square labeled 28710).
By reverse-engineering the die, I created the schematic below.
It is an op-amp, measuring the distinction between two inputs (one tied to ground).
The 2 JFET (Q12/Q13) transistors are configured as a old differential pair circuit.
A mounted fresh (from the fresh judge Q8/Q6) is fed into the transistors, and whichever transistor has the upper input will lunge nearly the total fresh. The consequence is amplified by Q5 for the output.
Besides the op-amp circuitry, the chip contains a reset circuit to discharge the capacitor sooner than expend (Q1/Q2).
(The inner capacitor C1 stabilizes the op-amp; the mixing capacitor is external.)
Schematic of the optical chip. Click on for a increased version.
The energy driver chip
Next, the energy chip drives solenoids and the motor to activate the camera’s mechanisms.
The excessive-fresh energy transistors (the golden triangular shapes) absorb most of this chip.
Smaller transistors below make the lend a hand an eye on circuitry.
My reverse-engineered schematic reveals that the chip has three parts.
First, a straightforward inverter. This doubtlessly interfaces the good judgment chip to the motor lend a hand an eye on board.
Schematic of the inverter in the energy driver chip. Click on for a increased version.
Second, a excessive-fresh driver. This makes expend of the kindly energy transistor at the left of the die. This doubtlessly drives a solenoid.
Schematic of the driving force in the energy driver chip.
Ultimately, a excessive-fresh driver with a separate circuit for the solenoid lend a hand fresh.
(That is, the solenoid is pulled into relate with a excessive fresh, after which held in that relate with a decrease fresh.)
This makes expend of the kindly energy transistors at the dazzling of the die. There may be a single gracious transistor below the necessary “triangle” of transistors; that transistor is for the lend a hand fresh. This circuit makes expend of separate energy and ground pads from the remainder of the chip.
Schematic of the driving force/lend a hand circuit in the energy driver chip.
The driver circuits are more advanced than I would maybe well additionally attach an relate to, using fresh sources and fresh mirrors.
Maybe this manufacture minimizes standby fresh expend.
The good judgment chip
The camera is controlled by a advanced good judgment chip
that controlled the timing of the many mechanisms, working the motor and solenoids.
It needed to tackle four totally different expend conditions: ejecting the protective quilt sheet when a movie bundle became once inserted,
taking a photo, taking a photo with the flash, and ejecting an empty movie bundle.
This chip became once made from Integrated Injection Logic (I2L), an vague 1970s good judgment family featuring excessive density and low energy.
For the explanation that camera ran off a battery in the movie pack, minimizing energy consumption became once a necessary factor.
At the time, I2L became once a appropriate preference for dense, low-energy circuitry, though it became once rapidly overtaken by CMOS.
Texas Instruments did relatively a few trend with I2L, including digital detect chips and the 76477 sound chip
so it be no longer hideous that they chose I2L for the camera chip.
I2L gates would maybe also be packed collectively at excessive density, as confirmed below. Every vertical grey rectangle is two transistors (one above the horizontal centerline and one below), comparable to two gates.
The chip has shrimp or no wasted dwelling, in particular when compared to TTL good judgment, which became once repeatedly ragged at the time nonetheless required multiple
transistors and burly resistors for every gate.
Closeup of the good judgment chip.
I2L is somewhat complex to brand since
an I2L gate has one input and multiple outputs. How can that work?
The schematic below reveals an I2L gate, with one input and three outputs.
In overall the fresh from the injector (ICC) activates the output transistor, pulling the output low.
However if the input is low, the output transistor turns off and the output will be excessive.
Thus, the gate inverts the input. (You may be in a obtain to think the injector as a pull-up resistor on the input.)
Implementation of an I2L gate. Sign that it has a single input and multiple outputs. Icc is the injected fresh. From “Integrated Injection Logic: A Bipolar LSI Methodology”.
For the explanation that circuit above has a single input, it would maybe well additionally appear like merely an inverter.
However by wiring several signals collectively at the input, you obtain an AND gate “for free”:
if any signal is low, this will additionally pull the wire low, and in any other case the signal is excessive.
This is named “wired-AND”. The wired-AND input to the I2L inverter leads to a NAND gate.
One problem arises with wired-AND:
if you happen to join an output to bigger than one wired-AND, the whole lot will get shorted collectively. The solution is to dangle multiple outputs from the inverter.
Thus, every I2L NAND gate has a single input and multiple an identical outputs.
Within the scheme below, the outputs from numerous gates (A and B below) are linked collectively and fed to the input of an I2L gate, atmosphere up a NAND gate.
Map of a NAND gate conducted in Integrated Injection Logic (I2L). From “Integrated Injection Logic: A Bipolar LSI Methodology”.
The transistors in I2L dangle multiple collectors, which would maybe well additionally appear abnormal,
nonetheless the scheme below reveals how they’re constructed.
Every collector has an N relate (pink) with a P relate (tan) below for the rotten, and one more N relate (inexperienced) at the bottom, forming an NPN transistor.
The multiple collectors are built by atmosphere up multiple N regions.
Physically, the injector PNP transistor is merely a P relate for the emitter, reusing the emitter and rotten’s N and P regions; this makes the injector more compact than a “beefy” transistor.
Die photo and lunge-share scheme of an I2L gate. The transistor rotten, collectors, and emitters are labeled alongside with the fresh injection.
I haven’t reverse-engineered this chip but. I imagine that it contains an oscillator and a chain of flip flops for timing,
to boot to a comparator for the light stage and a few miscellaneous lend a hand an eye on good judgment.
Whereas the electronics of the SX-70 camera are no longer spectacular by contemporary standards, they were lowering edge at the time.
They made the SX-70 easy to operate by facing the exposure and timing mechanically.
Texas Instruments split the electronics at some level of three chips: a precision JFET op-amp with a photodiode,
a excessive-fresh energy driver chip, and a advanced good judgment chip using dense, low-energy I2L good judgment.
Unfortunately, modern technology wasn’t ample for Polaroid.
The firm declined after competitors from Kodak, the costly failure of the Polavision instantaneous dwelling movie intention, and
the rise of digital cameras.
Polaroid declared bankruptcy in 2001 and the firm became once broken up.
The SX-70 has viewed a resurgence in popularity, with movie and cameras sold by polaroid.com, which got
the Polaroid name in 2017.
Apply me on Twitter @kenshirriff for more posts. I additionally dangle an RSS feed.
Because of the Joaquín De Prada and Peter Kooiman of openSX70 for providing the chips and John McMaster for decapping them.
The openSX70 project is constructing extensions to the SX-70 camera.
For more in regards to the SX-70, detect the attention-grabbing and quirky 10-minute movie below, which markets the SX-70, explains expend it, and discusses the inner
This movie became once made in 1972 by the infamous designers Ray and Charles Eames.