Delay ON, using a 555 and 12v motor

[cowboybob]

Bob,
I have a question for you and I am sure you did it your way for a reason, but I wanted to know why to better help me understand circuits.. Take a look at my .pdf... What would happen if we eliminated the lower 555 and tied the diode in this way?

Good question!

Woke up this AM wondering the same thing.

With the 555 configured as a "one-shot" monostable, there is the need to either re-trigger the circuit or power it down to effect a reset.

I haven't breadboarded these circuits (SIM only) but I'm confident they work. My concern was the transition time of the DPDT switch (the "off" period during the CW and CCW selections). I am uncertain that there would be an adequate power-off period for the 555(s) to "reset" and again produce the delay pulse.

I tried this in the SIM:

It took a minimum 150ms Timed Switch duration (thereby turning off power to the rest of the circuit) to effectively reset the 555. Any shorter duration failed to produce the reset. Of course, some of this time may have been a result of the inherent SIM speed. I don't know thw transition speed of the DPDT switch. I'll test a real switch's period later this morning.

Anyway, in the meantime, there's certainly no harm in wiring it as you suggest and trying it out.

 

[cowboybob]

Jeremy, your modified circuit won't be able to retrigger (the 1uF capacitor on the trigger pin will remain high after the initial relay activation). You could try something like the following mod. It relies on the DPDT switch being a break-before-make type, which will allow a small low pulse to be fed into the trigger pin when the motor hits an endstop and toggles the switch.

Interesting concept, dougy83. I understand your intent.

I assume you meant to leave the link from D1 to the Vdd rail of the 555, since otherwise there would be no power applied to the 555 circuit.

That toggle transition you wish to exploit might work except that during that transition the 555 also loses power, pretty much making the low going (see below) trigger generated ineffectual. It's that power loss I was trying to exploit as well, except as a total 555 reset.

Also, keep in mind that the absence of a + voltage doesn't equal a "hard" negative transition (to ground, ideally) that would trigger the 555.

 

[JeremyA]

I got the parts I needed last night but did not get a chance to breadboard them yet. Couple questions.
1.) I had to use a .047uf cap for the 50n cap into the voltage. Since they are + or - 20%, I figured that would be acceptable.
2.) The potentiometer I got was a 1m taper. I place this at the 1m resistor mark right?
3.) I am having trouble understanding the timed switch with relay 2. When you have a moment, can you tell me what your thinking here? You might have to dumb it down for me because I am slow with terminology and basic circuit knowledge.

 

[dougy83]

I assume you meant to leave the link from D1 to the Vdd rail of the 555, since otherwise there would be no power applied to the 555 circuit.
That toggle transition you wish to exploit might work except that during that transition the 555 also loses power, pretty much making the low going (see below) trigger generated ineffectual. It's that power loss I was trying to exploit as well, except as a total 555 reset.

I wrote "12V" on the power to the 555 to indicate that it is powered continuously.

Also, keep in mind that the absence of a + voltage doesn't equal a "hard" negative transition (to ground, ideally) that would trigger the 555.

The pull-down for the trigger is provided by the 10k resistor to ground.

 

[cowboybob]

I got the parts I needed last night but did not get a chance to breadboard them yet. Couple questions.
1.) I had to use a .047uf cap for the 50n cap into the voltage. Since they are + or - 20%, I figured that would be acceptable.

That's fine.

2.) The potentiometer I got was a 1m taper. I place this at the 1m resistor mark right?

Yes. keep in mind that a "taper" pot has a non-linear resistive sweep (designed as a volume control). All that means is that as you turn the pot's shaft, the resistance changes won't be linear. Only an issue if you decide to make a graduated dial for it.

3.) I am having trouble understanding the timed switch with relay 2. When you have a moment, can you tell me what your thinking here? You might have to dumb it down for me because I am slow with terminology and basic circuit knowledge.

That circuit was only temporary as part of the toggle transition time test. It is NOT necessary for your final circuit.

If your suggested circuit change works, great. If not, just revert to the original.

Like I said, I'll test a small DPDT toggle for transition times and let you (and me) know the results.

 

[JeremyA]

I wrote "12V" on the power to the 555 to indicate that it is powered continuously. The pull-down for the trigger is provided by the 10k resistor to ground.

Doug,
What does this mean to a layman?

 

[JeremyA]

Yes. keep in mind that a "taper" pot has a non-linear resistive sweep (designed as a volume control). All that means is that as you turn the pot's shaft, the resistance changes won't be linear. Only an issue if you decide to make a graduated dial for it.

So basically, if I wanted to make a dial with tick marks on it to identify times (or seconds), it would not be accurate?

 

[dougy83]

Doug, What does this mean to a layman?

I was just addressing bob's concerns re the circuit that I modified.

 

[cowboybob]

So basically, if I wanted to make a dial with tick marks on it to identify times (or seconds), it would not be accurate?

Yes and no. If the pot you have is the RS model 271-211, then it is a linear taper pot, allowing you to simply evenly divide the arc of its movement with your tick marks. If the pot you bought is not linear (therefore logarithmic, or audio), you'll have to test the timing positions and mark appropriately.

dougy83,

I stand corrected, with your noting of your power explanation. Your mod will work if the toggle transition time is short enough. In the SIM, 150ms was too short, 200ms worked. We'll just have to see with whatever JeremyA uses for the DPDT switch.

 

[JeremyA]

1.) Yes, Radio shack only had 2 left and they were the only ones to choose from, so this has to be the potentiometer. So it is linear.

2.) DPDT Mini Toggle Switch
Model: 275-663 (Here is the toggle switch I have)

 

[dougy83]

dougy83, I stand corrected, with your noting of your power explanation. Your mod will work if the toggle transition time is short enough. In the SIM, 150ms was too short, 200ms worked. We'll just have to see with whatever JeremyA uses for the DPDT switch.

Not sure what you mean. 150ms will be the same as 200ms will be the same as 20ms. The 100nF/100k filters out pulses longer than 11ms anyway. A very short trigger pulse is fine, and so is a very long one.

 

[cowboybob]

dougy83, let's just see what happens. I am continually amazed with 555 real world triggering responses...

JeremyA, here's a final schematic using dougy83's (considerably more elegant) mods (notice I removed the "start" circuit as well):

This circuit will have NO delay at power-up and, after re-reading your original 1st post, is what I now think you wanted in the first place.

 

[JeremyA]

You are correct. No delay at start up, only on toggle opposite. Thank you. I can't wait to try this out on my breadboard! I can't thank either of you enough for the thought that went in to designing this circuit. Means a lot to a noob.

 

[cowboybob]

Add this diode (which I forgot to include [a 1n4004 will do]) - it protects the 555 from the reverse current inductive "kick" of the relay when it is de-energized:

 

[4pyros]

What is going to revers the motor?
I assume you wanted it auto reversing right?

 

[cowboybob]

What is going to revers the motor?
I assume you wanted it auto reversing right?

The DPDT toggle switch. It's mounted such that it encounters a pin (rod, whatever) as it traverses a path, both ways, thus "toggling" it.

 

[4pyros]

The DPDT toggle switch. It's mounted such that it encounters a pin (rod, whatever) as it traverses a path, both ways, thus "toggling" it.

One at either end like limits?
Never mind I now see its on the motor trolly.

 

[JeremyA]

I haven't had much luck with the circuit. I have reworked it numerous times to make sure I am not doing something wrong, but I don't seem to be getting it. I will try to post some pics of the breadboard or something and maybe you guys can give me a few tips. I CAN hear the relay click after the toggle. When the pot is near 0 secs, its almost immediate, but it does not reverse. When I turn the pot for more time, it does delay for a short time, but then it starts clicking (Doesn't sound healthy..lol) and I smelled a bit of hot circuitry a few times. I don't think I burnt up the relay, but what do I know? lol.