Too seductive were those Efratom LPRO-101 on the bay, waving a flag "buy me".
Efratom LPRO-101, inside view
Not only the bargain offers in the bay, but also as a backup system for the hp58503B (GPS disciplined OCXO) made it necessary to have such a
As there is almost everything done, we may use the space and time (saved) to build an interface with some additional features.
An Atmega 8 reads some analog voltages like supply or lamp voltage and feeds an lcd with the data. The words "locked", printed
on a screen or display are more informative than just a led, which when not lit mayst be defective. Beyond that, it is the occasion to learn this ADC stuff, sleeping
(up to now) in the Atmega :-)
The circuit is straightforward. The power supply has been designed out of the case. This allows for battery powering, when necessary. As the
power consumption of the Atmega 8 and the lcd is low, a linear regulator generates the +5V out of the +24V. Regarding the output of the LPRO-101, it has series resistors
of two times 10kΩ inside, so we added a shunt of 4kΩ. This guarantees, that the analogue inputs of the Atmega stay below 2.5V. None of those
measurements are time critical, so running at 1 MHz is absolutely sufficient. The circuit was realised on a double sided pcb. In case the serial
comunication is used, the crystal may be adapted, depending on the datarate.
When switched on, the device draws about 1.5 A. The current then reduces to less than 500 mA, when the cavity is heated. (The lcd draws approx 100 mA). After
some minutes, the lamp voltage reaches its final value of approx. 8 V (depends on age of device, some others show 6 V) and the bite goes low, indicating
a "locked" status. With the maximum gate time of 100 s, we could not measure any difference to our hp58503B (GPS disciplined OCXO) as our reference.
The heatsink turned out to be slightly oversized :-)