inexpensive timing relay circuit

Thread Starter

strpdbas

Joined Dec 24, 2008
99
Two questions. First if I am reading the diagram correctly the varpot the center connection is not used, only the connections across the resistor right? Second, how easily could I increase the variable delay if I wanted to have a 10 second to 60 second delay? Would this require only a different pot or would other components need to be changed?
Don't do too much celebrating tonight, I need your mind to be clear!!LOL
Happy New Year.
Thanks, Joe
 

eblc1388

Joined Nov 28, 2008
1,542
Two questions. First if I am reading the diagram correctly the varpot the center connection is not used, only the connections across the resistor right?
That might be my oversight. You should connect the center connection to either one of the remaining terminals. Some VR comes with that done and have only two connections.

Second, how easily could I increase the variable delay if I wanted to have a 10 second to 60 second delay? Would this require only a different pot or would other components need to be changed?
You only need to change the values of VR(R3) or the capacitor(C8) or both. There is an online webpage that helps to calculate the timing.

Keeping R3 unchange but changing C8 to 470uF will give you max. of 52 seconds.

555 MONOSTABLE TIMEOUT CALCULATOR
 

Thread Starter

strpdbas

Joined Dec 24, 2008
99
Informative web site, thanks.
I am curious if this circuit could be designed to eliminate the need for mechanical stops and the limit switches? Perhaps an additional timing circuit to allow for variable run times, say 4-10 minutes.
Thanks, Joe
 

Thread Starter

strpdbas

Joined Dec 24, 2008
99
I am waiting for the balance of components to arrive.
Believe me you will be the first to know when the circuit is built.
I don't know if you received my last post but I have 2 questions. First, can the circuit be designed to eliminate the need for limit switches? I would like to be able to set the motor run time in a similar fashion to the adjustable time delay. If I do need to use limit switches could I use spst or would I need spdt wired like a 3 way light switch? I am having difficulty finding limit switches with momentary contacts. Thanks again for your help.
Joe
 

eblc1388

Joined Nov 28, 2008
1,542
First, can the circuit be designed to eliminate the need for limit switches? I would like to be able to set the motor run time in a similar fashion to the adjustable time delay.
You cannot eliminate the limit switches. You cannot depends on how long the motor is running to calculate if the end stops are reached. This will only work if the motor can be stalled when it reaches the end. I'll give an example.

If the end to end travel normally takes 15 seconds and you set the timer to 17 seconds, then for 2 seconds at the end of travel the motor would be running against the end stop and the controller is not moving. Can the mechanism or motor survive this?

Say for another scenario that the permissible rail travel distance is longer than the wanted motor travel distance. So it will take 30 seconds for the motor to travel from end to end but you then set up the motor to reverse direction every 15 seconds. You are effectively moving within a range in this allowable travel distance. You would think this would be OK but in the real world things will get worse. Frictions are not always constant. The travel distance of the "motor+controller" will slowly drifted to one end so perhaps after sometimes the motor WILL again hit one of the end stops and stalled.

If I do need to use limit switches could I use spst or would I need spdt wired like a 3 way light switch? I am having difficulty finding limit switches with momentary contacts. Thanks again for your help.
Joe
I do not understand your concern regarding limit switches.

All limit switches have momentary contacts(spst), some even have changeover contacts(spdt). A spdt limit switch can be used also as a spst one. You just leave out the connection to NC contact.
 

Thread Starter

strpdbas

Joined Dec 24, 2008
99
Perhaps I misunderstand the function of spst vs momentary in regard to a limit switdch. I assume the contacts of a spst momentary switch are only closed while the lever is depressed against the stop, and open when released. If the switch is spst don't the contacts close and latch until the lever is depressed again, just like a light switch, or do all spst limit switches have momentary contacts? I appreciate the info about Radio Shack I will check their web site as the switches I need must be mounted through a hole in the control box keeping the connections concealed. Thank you
 

eblc1388

Joined Nov 28, 2008
1,542
Perhaps I misunderstand the function of spst vs momentary in regard to a limit switdch. I assume the contacts of a spst momentary switch are only closed while the lever is depressed against the stop, and open when released.
The assumption is correct. The contact of a limit switch only closes when the switch lever is depressed but returns to open if the lever is released.

If the switch is spst don't the contacts close and latch until the lever is depressed again, just like a light switch, or do all spst limit switches have momentary contacts?
Only very special types of limit switch will latch after operation. Most ordinary limit switches will reset, unlike a light switch.
 

Thread Starter

strpdbas

Joined Dec 24, 2008
99
Thanks for the clarification. I was getting confused because when I would search online for limit switches some would state "momentary" while others would state "snap action". I was concerned that if the contacts did latch the switch would not reset the circuit because the contacts would open the next time it hit the stop. So any spst limit switch will have momentary contacts, and must be wired in a normally open position.
I can't wait to get this thing up and running.
Than you very much.
Joe (Strpdbas)
 

Thread Starter

strpdbas

Joined Dec 24, 2008
99
I completed the prototype board but it is not working correctly. The motor runs, but pressing the limit switch neither stops nor reverses direction. I would appreciated it if you would review my board layout for errors. The last time I sat down with a soldering iron was when I built a Heathkit receiver in 1975. I don't know if the were available in GB, but they used to make DIY kits for a number of electronics products. I don't know how old you are so you may be too young to remember them.
Thanks again for all of your help, Strpdbas
Controller.gifView attachment Ckt.pdf
 

SgtWookie

Joined Jul 17, 2007
22,230
2N7000 MOSFETs have a maximum Rds(on) of 5 Ohms when Vgs=10 with a current of 500mA.

I do not see any bypass caps on U1 (0.1uF) nor the 555 timer. The latter will need both a 100uF and a 0.1uF cap to ensure a reasonable amount of stability.
 

Thread Starter

strpdbas

Joined Dec 24, 2008
99
Here are both sides of the board. The layout I previously sent is a little different, but the connections are the same.
PICT0001.JPG

PICT0006.JPG

PICT0007.JPG
 

eblc1388

Joined Nov 28, 2008
1,542
Your board design seems to be in order.

When you said it did not work, did you see any response from the relays if you operate the limit switches?

What you can do is to measure and post the following voltages. All measured with the black(-ve) lead of the voltmeter connected to 0V or common of the circuit and with DC power ON. So be careful not to short circuit anything or touches the live AC connections.

1. Pin 14 of CD4013B
2. Pin 8 of 555
3. pin 4 of CD4013B, limit switch not pressed
4. pin 4 of CD4013B, limit switch pressed
5 & 6. Same as 3 & 4 but on pin CD4013B 6 instead
7. pin 1 of CD4013B
8. pin 2 of 555
 
Top