Use 555 timer to output same interval as user presses a button

Question

I was curious if such a circuit already exists tentatively using 555 timer chip (I know I can do this with a micro controller so please don't suggest that). Basically I need following functionality: - by default output is low - user presses a button for random time interval T - once button is released the output goes high for same time period T then goes back to low. Any ideas or circuits are highly appreciated. Thanks !

• TonyM: You could do it with a comparator, using an RC charging when you press the switch then a separate R to discharge C for the same time. Switch would have to defeat the comparator output while pressed. Comparator delivers the signal. But it's a bit rubbish and limited/inaccurate for a dozen reasons. Any reason you can't/won't use digital? Is it just a fun idea to investigate?
• M.Ferru: Instead of doing the circuit for you, I invite you to download LTSpice, read the 555 datasheet and try yourself.
• vicatcu: This question deserves this: (╯°□しろいしかく°)╯( ┻━┻
• s4581218: I already built a circuit using an RC charge/discharging controlled by a SPDT switch. Problem is getting timing right. The RC curve is flat near VCC when charging and flat near GND (0V) when discharging, thus those regions are susceptible to noise. I then thought that this RC circuit needs to operate in mid-region of RC curve where it is closer to linear. Thus I remembered the 555 timer where the thresholds are 1/3 and 2/3 VCC, hence the idea to use it. I could probably use a dual-op amp or something to do the same. Just checking I am not reinventing the wheel. Already using LTSpice , Thanks.

Answer 1

Yes, you can set a '555 to oscillate, and use logic to (1) count UP at the oscillation frequency while the button is held down, (2) count DOWN and set the output pin high, when the button is released, and (3) reset the output pin low when the count-down reaches zero.

One or more 74HC191 counters would be suitable for the count function.

The '555 is intended to make a fixed time interval, so accumulating a pulse count (clocking) is the easy way to implement a variable time delay.

• s4581218: I am trying to keep component count to a minimum this has to be a sub-miniature circuit. Thanks , great idea.
• D Duck: If the goal is min parts count, use a microcontroller. In one you have an oscillator and a counter and available in a 8 pin package. But you don't or won't use one of those. It's a bit like wanting to build an iPhone, but you can only use electromechanical relays and then when a design is presented, complaining that it won't fit in your pocket and that the battery life is minimal.