Is it safe to use a CMOS IC? For instance I'll be using a CD4017 in this circuit:
http://www.555-timer-circuits.com/police-lights.html
Will it still function correctly? Any further modifications needed?
Electronic – Using CMOS IC in a TTL circuit
cmos
Related Solutions
Imagine a load resistor (say 10K) from output to Vdd, and make X_1 and X_2 low. Ideally, output should be 0.0V. You can figure out what it is (not 0V).
Now imagine a 10K load resistor from output to GND and make X_1 low and X_2 high. Ideally, output should be Vdd. You can figure out what it is (not Vdd).
simulate this circuit – Schematic created using CircuitLab
The easiest way to deal with a digital signal like that is to feed it to a BJT or MOSFET with a pullup from the processor voltage. For example:
simulate this circuit – Schematic created using CircuitLab
The diode allows the input to go negative as well without damage to the processor or transistor. Brief spikes of +/-100V won't hurt anything, regardless of the processor power state. Cost is pennies, at most.
Best Answer
No, that's not guaranteed to work.
TTL low level outputs are no problem, but the high level may be as low as 2.7 V, and that's too low for a CMOS high level input.
Instead of a CD4000 series device I would use a function and pin compatible 74HCT4017. 74HC is High-speed CMOS (HCMOS), which is the technology most used today, the CD4000 CMOS is really outdated. 74HCT is the same, but with TTL compatible I/O (hence the "T").
Further reading
Logic signal voltage levels