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 only to the metering system - the counter is actually digital with an 8-bit microcontroller (Intel 80C31 @ 6MHz), firmware with a very robust algorithm, stored on 27C32 EPROM, an AD7524 8-bit DAC to drive the metering system and multiplexed read of the ranges and configuration dip-switches. Functionality-wise it is way more advanced and more flexible instrument than Ludlum Model 3, with a lot more features, measurement units and ranges and far more sophisticated circuitry.

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

I decided to add a "visual click" with a LED, just like the one found on the more modern counters. As I mentioned the electrical design is beautiful and it wasn't difficult to figure out how to implement this mod.

The good news is that it is super-easy to mount the LED from a mechanical stand point as well - since the metering system has a backlight feature there is absolutely no need to modify the case - this is a slick "no drilling", very unobtrusive mod.

The LED is mounted behind the metering system and projects the light on the white, semi-translucent meter backing, using it basically 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, junction is heat-shrink insulated and then mechanically attached with hot glue to the PCB, just behind the metering system. The LED Cathode is connected with 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, providing mechanical rigidity.

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 dabs to the PCB and uses the notch of the edge connector to transition PCB sides.

This schematics shows how the LED is integrated in the electrical 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 not only solves this problem but takes advantage of the AES-1 ability to divide high click-rates with user-selectable division factor (when a scintillating detector is used for example).

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

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