Friday, 27 November 2015

Teardown: Grass Instruments DC Driver Amp & Polygraph Integrator

In this video i begin looking inside some of the modules making up the Grass Instruments Polygraph.

Teardown: Grass Instruments Polygraph / EEG

This will be the beginning of a number of articles featuring the latest bit of hardware for teardown.

Although i had not intended to buy this particular item i managed to get a good price from the seller. Having previously seen it when i picked up another teardown item i knew it was loaded with lots of control knobs that would be worth money to the right people, not to mention all the metal so i took a gamble on buying it thinking it may pay for its own teardown.

I had to pickup in my van which was only just big enough and probably totalled around 300kg.

So the items i have consist of two polygraph machines the Grass Instruments Model 7D and the Grass Instruments Model 78 with chart recorders (oscillographs). There is also an additional chart recorder.

A polygraph is an interesting term, partly for the fact that it is commonly associated with lie detection, a procedure mostly used in the USA. A polygraph machine is not necessarily a lie detector. Lie detection is a technique that just uses a polygraph to monitor certain reactions to various questions, the operator is the one who decides if the person is telling the truth.

For the most part polygraphs are used in EEG (electroencephalography) which is monitoring electrical activity in the brain and also for sleep disorders amongst other medical uses. The polygraph simply takes very low amplitude signals from sensors attached to the skin, pulse rate, brain activity or other sensors that could monitor breathing or muscle responses. The signals are passed through amplification and filtering modules which then drive mechanical pens on a chart recorder. Modern machines would simply digitise the signals in a small box to be displayed on a laptop to dispense with the large machines featured here.

These polygraph machines were manufactured around the early 1970s, i say 'around' as many of the modules have slightly different dates of manufacture but seem to date between 1973 and 1981. They are from the Grass Instrument Co founded by Albert Grass in 1935.

Friday, 20 November 2015

Minitron [Numitron] 3015F Vintage Incandescent 7 Segment Display

I recently found these Minitron 3051F displays in some equipment i dismantled.

The Minitron is a 7 segment display with a decimal point that uses small incandescent filaments made from Tungsten to form the segments. They are an evolution of the Numitron which works on the same principal but are packaged in the old 'valve' style glass packages. The minitron is packaged in a more modern 16pin DIP form like LED based 7 segment displays but with a borosilicate glass cover. Note the decimal point is an 8th filament but is partially masked off.

Operating these is very simple, the recommended operating voltage is 5v with each filament drawing around 8mA at 5v so can easily be driven directly from microcontrollers (making sure your accounting for the cold resistance and resulting current spike when they first turn on). I measured the current draw at 5v with all the segments on (including the decimal point) and it totalled 67mA. They have a surprisingly long life of around 50,000 hours at 5v.