UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Retro computer homebrew: Orton Karenbak-1 computer

Argus25 · 7th Jan 2019, 2:47 am

I'm not sure if this has any application in your project:

One very effective simple memory, to make with discrete parts, which acts as a memory for an analog voltage, which could be derived from a DAC as one source, is to use a single mosfet in conjunction with a reed relay, with a small capacitor of a low leakage type, 0.01uF to 0.1uF or similar in the gate and use the mosfet in a follower mode.

You can then get the output of that and ADC it back to a digital byte etc. Due to the insignificant gate current this can remember the voltage value for many days, even with a small capacitance. This trick was used in early color video cameras to remember the offsets for a white balance adjustment. When I first saw it I thought it wouldn't be any good as a memory, but it really was. It could just be used for a single bit storage too.

The capacitor, reed relay connection and Fet's gate connection are lifted free from the pcb to reduce leakage. Small reed relays, coil driven, easily operate at 300Hz too, and the size of the capacitor can be reduced to allow quick charging.

7th Jan 2019, 2:47 am	#15
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Retro computer homebrew: Orton Karenbak-1 computer I'm not sure if this has any application in your project: One very effective simple memory, to make with discrete parts, which acts as a memory for an analog voltage, which could be derived from a DAC as one source, is to use a single mosfet in conjunction with a reed relay, with a small capacitor of a low leakage type, 0.01uF to 0.1uF or similar in the gate and use the mosfet in a follower mode. You can then get the output of that and ADC it back to a digital byte etc. Due to the insignificant gate current this can remember the voltage value for many days, even with a small capacitance. This trick was used in early color video cameras to remember the offsets for a white balance adjustment. When I first saw it I thought it wouldn't be any good as a memory, but it really was. It could just be used for a single bit storage too. The capacitor, reed relay connection and Fet's gate connection are lifted free from the pcb to reduce leakage. Small reed relays, coil driven, easily operate at 300Hz too, and the size of the capacitor can be reduced to allow quick charging.