Turbo-display for ZX81

simple project no internal changes needed
Do you remember those days XT's and AT's had a "TURBO" switch and LED??
This was a very expensive new feature and you hat to bend down under your table to use it or to look for the LED.

Our good old zeddy had the turbo-mode very much earlier. And what modesty!! Sir Clive called the normal-mode "SLOW" and without any extra charge, we all got the turbo-mode for our ZX81 which is called "FAST". And we can operate it from keybord or even by a simple command in our programs, no extra switch is needed for this feature.

Here you get the missing TURBO-LED for your ZX81!

How does it work?
The 555 timer-circuit works as a monostable, which will be triggered by the /NMI-signal. The PNP-transistor will retrigger it, as long as NMIs are detected within a period, depending on the time-constant of 390K ohms R and 100nF C.
The green LED will display "everything is ok". But if the ZX81 changes to FAST-mode, /NMI will be constant HIGH. Now the red-LED will work and show us that our ZX81 became dangerous fast.

You can connect the tree wires to an edge-connector:
+5V: pin 1B, 0V pin 4B or 5B form the underside
/NMI from pin 12A at the component side

If you replace the LEDs by small relais, you have a very simple "output-port"

pocket-tester for ZX81
Put this circuit together with three 1,5V batteries or four 1,2V NICAD-cells in a small box. Add a RESET-switch, and your ZX81-pocket-tester is ready for use on fleamarkets!
Yes, it's good as a very simple test-equipment.
After you reset a ZX81, in the first moment, it will switch-off the NMI and perform the memory-test. After that, the NMI-generator in ULA is switched-on.
After each RESET the red-LED must be on for a short time and then the green-LED must be on. Do you know such a simple-pocket-tester for any other computer?????