Hi all

Is it possible to select a 555 between 2 modes?

I have a 555 controlling a relay in monostable. This relay controls what I see on my Raw LCD Dash monitor allowing me to select between 2 sources of video. Front or rear cameras.

But ... I would also like for it to operate in Astable mode. Cycling the relay every 10 to 15 seconds.

Is this possible? If a 555 can not be used for this, that is fine.

TONY

 

You can do this. In principle, you connect the trigger input of the '555 to the discharge and threshold inputs instead of to whatever you were using to trigger the monostable. The details depend on just how you have it connected at the moment and exactly what you want it to do in astable mode - what time on and what time off.

Can you give us a schematic of your present circuit, please.

 

Can you give us a schematic of your present circuit, please.

I have attached my monostable circuit I use. Works great. I use the C3 & R16 to prevent De-Bouncing when triggering the circuit.

TONY

 

Your circuit will already work as an astable if you keep the button pressed though it will probably be too fast. It will change state roughly 10 times a second. You could increase the values of R16 and C3 to slow that down to suit your needs.

 

You could increase the values of R16 and C3 to slow that down to suit your needs.

Thank you ... I increased C3 to 22uF Electro style cap and R16 to 10K ohms. Works in the simulator just like you said. When button is held close. Seems to be oscillating in 5 sec intervals with simulator. I tried to increase the values to increase the time between switching. But my SIM didn't do anything. Just stayed in one state. ... Off to the breadboard for further testing!!

Thank you so much!!

TONY

 

That's a circuit of a Flipflop using a 555, not an Astable circuit. The output will only change state on each press of the button.

 

That's a circuit of a Flipflop using a 555, not an Astable circuit. The output will only change state on each press of the button.

As the TS has confirmed, it becomes an astable if you keep the button pressed as the feedback to trigger and threshold comes from the output.

 

Using my Breadboard. I have the following results when adjusting C3 and R16

2200uF
5.5K
Off 10sec
On 15 Sec

2200uF
6.8K
OFF 14 sec
On 22 Sec

2200uF
8.2K
Off 17sec
On 35 sec

I would like for the on and off time to be more equal to one another. And with C3 value being so high. I can NO LONGER push the button to be monostable. .. I did NOT try to pulse the trigger from straight ground to see if monostable would work. I will try that tonight.

TONY

 

To make the times more equal you should use the cmos version, LMC555 or TS555.

What you could do is to keep the original circuit with R16 and C3 and the button for the monostable version then add another resistor and capacitor connected the same way, but with their junction connected by a new astable switch to pins 2 and 6.
Keep the new resistor at 100k rather than using the rather low values you were using for R16.

 

To make the times more equal you should use the cmos version, LMC555 or TS555.

I will have to get a LMC555 to test. I do NOT have those

To make the times more equal you should use the cmos version, LMC555 or TS555.

What you could do is to keep the original circuit with R16 and C3 and the button for the monostable version then add another resistor and capacitor connected the same way, but with their junction connected by a new astable switch to pins 2 and 6.
Keep the new resistor at 100k rather than using the rather low values you were using for R16.

I believe I know what you are referring too. I attached a new circuit drawing to this post.

TONY

 

You would be better off using a CD4093 Quad Nand gate, you can make an oscillator using one gate, and a flipflop using two gates, that give you a gate left.

 

I attached a new circuit drawing to this post.

No you didn't

 

He did, he's just deleted it!!!

 

No you didn't

How did I forget that. Sorry I missed that ... Now it's attached! ...

 

Yes, that's the way. I have just two comments:
1. The LED will be very bright. It's going to be running at about 20mA. Your original diagram includes an LED across the relay and that's a better place to put it. Putting it on the '555 output, especially at such a high current will still upset the on/off time matching.
2. It will work with 8k2/2200uF, but it will probably be better with, say, 82k/220uF, but if you don't have those values then OK.

 

Yes, that's the way. I have just two comments:
1. The LED will be very bright. It's going to be running at about 20mA. Your original diagram includes an LED across the relay and that's a better place to put it. Putting it on the '555 output, especially at such a high current will still upset the on/off time matching.
2. It will work with 8k2/2200uF, but it will probably be better with, say, 82k/220uF, but if you don't have those values then OK.

I use a 2K or 3K for the LED on the 555 output in reality ... but a 500 in the sim only. Otherwise the sim does NOT operate the LED. Go figure. :/

I have those value and will try them on the breadboard. I will share my findings.

Thank you!

TONY

 

Hi All

I have attached my latest schematic and it does work. The Push button still toggles the 555 and the switch starts the timer.

I have played around with the values of R5 and C2

R5 = 5.5K
C2 = 2200uF
On 15 sec
Off 15.5 sec

R5 = 5.7K
C2 - 2200uF
On 16 sec
Off 16.7 sec

R5 = 8.2K
C2 = 2200uF
Only turned on and off Once. Then always stayed on.
I do NOT know why.

R5 = 10k or greater
C2 = 200uF, 330uf, 470uF or 2200uF
Output Never changed states.

Lowering the values of either R5 and C2 shorten the time it took to change states.
Good news is, the Push Button always worked!

At this point ... I think 15 or 16 seconds is the best I can do. I am NOT sure what else can be done.
Is there entirely different setup I can use to achieve the same results, but with longer timer on and off time (30 seconds)?

TONY

 

You need to increase R1 and R2 to a large value, say 1MΩ.

 

You need to increase R1 and R2 to a large value, say 1MΩ.

I will try that next. Thanks.

TONY