How design a circuit to control a switch?
This is a simple circuit that may work. As noted in the comment, however, AFAIK, TTL does not work on 0.8 or 1v. So whether this works or not will depend on what circuitry you're actually using is. However, it can be modified to suit your needs.

In short - it will turn on if the signal is higher than around 0.65v, and off when the signal is lower than around 0.65v. Some TTL outputs will not go low enough with a logic zero. And I imagine, if your "TTL" circuitry has a positive supply of 0.8v, the output may not reach 0.65v on a logic 1. You should take measurements of a logic 0 and logic 1 output, and then a circuit could be designed to accommodate it. (Also, your logic circuit may not "pull high" at all. Usually the case for TTL.)

Depending on your input signal, you might have to modify the values of R1, R2 and R4.

Last but not least, you have not said how much current your load may draw. The circuit I have given should power a load that draws up to about half an amp. 







simulate this circuit &ndash; Schematic created using CircuitLab