Wednesday, March 3, 2021

Eberline ASP-1 LED modification - "visual pulse indicator"

Eberline ASP-1 is one my favorite "80s era" Geiger Counters. 

The electrical design is elegant, downright beautiful. ASP stands for "Analog Smart Portable" but "Analog" refers to the metering system only - the meter is actually digital with an 8-bit microcontroller (Intel 80C31 @ 6MHz), firmware with a very robust algorithm, stored on 27C32 EPROM and an AD7524 DAC to drive the metering system. Functionality-wise it is way more advanced and more flexible instrument than Ludlum Model 3, with a lot more features, measuring units and ranges and more sophisticated digital circuitry.

The speaker of the unit is not terribly loud - it is more of an acoustic air-tube type headphone transducer than a proper speaker.

I decided to add a "visual click" with a LED, just like on the more modern counters. 

The good news is that it is super-easy to mount the LED - since the metering system has a backlight feature there is no need to modify the case at all.

The LED is mounted behind the metering system and projects the light on the white, semi-translucent meter backing, using it as a "projection screen".

When the LED is off, the metering system looks just like before.

Behind the scenes. The Anode of the LED is connected to a wire, heat-shrink insulated and then mechanically attached with hot glue to the PCB, just behind the metering system. The LED Cathode is connected thru a 220 Ohm resistor to the GND lead (the left lead on the picture) of the Speaker switch and this is also a second anchor point.

The Anode lead wire of the LED is connected to junction point between R138, R139 and pins 1,2,4 of A104 (CD4001B) on the bottom of the PCB.

The wire is secured with hot glue to the PCB and uses the notch of the edge connector to change sides.

This schematics shows how the LED is integrated in the circuit.

The mod is using the speaker circuit to pulse the LED as this circuit is designed to provide long enough pulses (~2 ms) for the speaker, using a free-running 2kHz generator and a mono-stable trigger. 
The pulses behind the pre-amp which are counted by the MCU are extremely short (just a few uSec) and they will not light up the LED for a long enough time to be visible - using the audio circuit solves this.

The LED will light up permanently if there is an Alarm condition (Overload).

Turning the Audio feature off will disable the visual pulse indicator as well but the Alarm will still work as designed and the LED will still respond to an alarm condition.