Monostable 555 Timer Application Question

Thread Starter

mechengineer61

Joined Feb 4, 2020
4
I'm pretty inexperienced when it comes to electronics and circuits so bear with any stupid questions I may have.

I'm working on a simple DIY project. It is essentially just a box and the high level idea is that when the box is closed, a circuit is completed where it then automatically executes some functionality. All I need it to do is light some LED's inside the box for ~300 seconds then automatically turn them off until I open the box lid (break the circuit) and close it again (complete the circuit).

I've done some research on the cleanest and simplest way to implement this and a 555 timer in monostable seemed like a good solution. However pretty much every example of 555 timer monostable operation I've seen uses a push button as the trigger. I do not want this as I want the lights to turn on as soon as the box is closed, without the need to push a button. Ideally I'd have the trigger and voltage supply share a line. Is there a better or easier way of accomplishing this? It seems like I can use a capacitor right before the trigger to provide an initial pulse but what happens when the capacitor is left connected to the voltage supply for multiple hours?
 

dl324

Joined Mar 30, 2015
10,072
Welcome to AAC!

A simple way to trigger the one shot is to have the lid close a switch that is AC coupled to the trigger.

Another option is to use a light dependent resistor (LDR) to detect the closure, but that would be affected by ambient light levels and would require a comparator to get a crisp transition point.
 

Thread Starter

mechengineer61

Joined Feb 4, 2020
4
Welcome to AAC!

A simple way to trigger the one shot is to have the lid close a switch that is AC coupled to the trigger.

Another option is to use a light dependent resistor (LDR) to detect the closure, but that would be affected by ambient light levels and would require a comparator to get a crisp transition point.
Thank you for the response! I feel like that unfortunately may not be an option for me as this will be run off batteries and thus have a DC power supply.
 

crutschow

Joined Mar 14, 2008
24,403
How about a microswitch operated by the lid?

But getting 300 seconds from a 555 is problematic due to the leakage of typical large electrolytic capacitors coupled with the high timing resistor value needed for such a long delay.
A CD4060 based timer, such as here, might be better, since it can give long time delays with a small capacitor value.
It is a CMOS device so will draw very little quiescent power from the battery.
 

MrChips

Joined Oct 2, 2009
20,329
A number of clarifications are required.

When @dl324 said AC coupled, this has nothing to do with AC power versus DC power. I will explain further below.

First of all, be aware that there are different 555-timer ICs.
NE555 and LM555 are higher current models while LMC555 and TLC555 are low power CMOS chips.
You can choose however you wish to trigger the timer, with a push-button, LDR (light dependent resistor), micro-switch or any kind of electrical signal.

What @dl324 refers to as AC coupled is a signal derived from any of the above circuits so that the 555-timer is triggered on the closing of the box or the opening of the box. The choice is yours depending on how you configure the circuit.

Having said that, the 555-timer circuit is a poor choice for any time delay longer that a few seconds. This is because the RC timing circuit is not very repeatable for large values of capacitance C. You can either add a counter chip in order to count seconds or you can have a counter chip that includes the timer function.

For ultra low power applications, sometimes an MCU solution is the way to go.
 

Rich2

Joined Mar 3, 2014
151
I made a 10 minute off delay timer with a 555 circiit. It's not accurate I suppose but it doesn't need to be for my use. It switches a tracker off 10 minutes after the car ignition goes off.

Try your circuit with a 220uf capacitor and a 2M ohm resistor in parallel with it (to discharge it and set the time)

If you're putting a capacitor in series with the trigger switch, that will need a high value resistor in parallel so it discharges and resets for next time.

Nothing will happen to it if it's connected to the voltage supply for multiple hours, apart from charging up.
 

eetech00

Joined Jun 8, 2013
1,838
Hello

The TS hasn't stated how accurate the timing has to be.
If it doesn't need to be really accurate, a 555 can be used wired in a standard, "power-on reset" monostable configuration.
Power to the chip would be removed by a NO micro switch on the box lid. When the lid is closed, power is applied to the timer and the timer runs for 300 seconds. When the lid is opened, power is removed from the timer. If the lid is closed again, the timer runs for another 300 seconds, etc..

eT
 

Thread Starter

mechengineer61

Joined Feb 4, 2020
4
Hello

The TS hasn't stated how accurate the timing has to be.
If it doesn't need to be really accurate, a 555 can be used wired in a standard, "power-on reset" monostable configuration.
Power to the chip would be removed by a NO micro switch on the box lid. When the lid is closed, power is applied to the timer and the timer runs for 300 seconds. When the lid is opened, power is removed from the timer. If the lid is closed again, the timer runs for another 300 seconds, etc..

eT
The timing does not need to be super tightly controlled. I'd say 300s +/- 15s would be acceptable. It is more of a minimum time requirement so I can just add some margin to ensure that the bottom range of the timing tolerance is still within what I would consider acceptable. However, whatever the time ends up being, I'd like it to be repeatable. So that there is a negligible amount of variation of timing between cycles.

The behavior you described for the opening and closing of the lid is basically exactly what I'm looking for.
 

crutschow

Joined Mar 14, 2008
24,403
whatever the time ends up being, I'd like it to be repeatable. So that there is a negligible amount of variation of timing between cycles.
I think for your requirements, a CD4060 timer, as I referenced in post #4, would be a better solution.
It would likely be more repeatable and stable, and allow the use of a much smaller capacitor, than a 555 design.
I can help you with that design, if you like.
 

Thread Starter

mechengineer61

Joined Feb 4, 2020
4
I think for your requirements, a CD4060 timer, as I referenced in post #4, would be a better solution.
It would likely be more repeatable and stable, and allow the use of a much smaller capacitor, than a 555 design.
I can help you with that design, if you like.
Yeah thanks a lot for this suggestion. I hadn't even heard of one of these before so this is very helpful. I'm going to do a little research into how they work and read through the page that you link but I may very well take you up on that offer for a little help!
 

crutschow

Joined Mar 14, 2008
24,403
I'm going to do a little research into how they work and read through the page that you link
You would use Version 2 of the circuit with the relay replaced by the LEDs (the transistor can directly drive the LEDs).
The "Range" connection would go to pin 6, which gives a timing range of 4min to 12min as determined by the adjustment of pot R4.

For your timing requirements you could use a smaller C3 capacitor value with the appropriate change in the "Range" connection.
For example, a reduction in the capacitor value from 10μF to 100nF (a factor of 100) would reduce the delay range also by a factor of 100, thus the "Range" connection would then be to pin 2 (4hr to 12hr range, now 2.4min to 7.2min).

The time delay is relatively easy to adjust, since you can look at one of the faster outputs to set the desired oscillator frequency with R4.
Each output is a factor of 2 different in time from the next in the sequence.
That way you don't have to wait the full 300 seconds to see if the frequency is correct.
 
Top