Want to build a 2 event delay timer controller

Yes, I can design a circuit for you that does what you want. I'm helpful like that (Actually it's pretty simple.)

Your description is good, and that's such a relief. We see some incredibly vague questions here on these forums. But I need to clarify several points.

Correct me if I'm wrong on any of this. You want to power the circuit from a 12VDC source that can supply 500 mA. You want two independent relays, to control two separate external devices. You have a completely isolated pushbutton or switch which is closed to trigger the circuit. When the circuit is triggered, the first relay needs to close for 5~8 seconds. After this time, the first relay opens and the second relay closes. One second later, the second relay opens. Then the circuit resets and is ready to be re-triggered.

Re the 5~8 second delay. How accurate does it need to be? How do you want to set it? Would a rotary potentiometer with approximate durations marked on a dial be enough? Would an overall accuracy of +/- 5~10% over temperature and age be good enough? Would you prefer a switch to select between several options?

What country are you in? Do you have an account with any electronics component supplier such as Digikey, Mouser, Farnell/Element14, Newark etc? What experience do you have with circuit construction? Have you used stripboard before? How do you plan to house the circuitry?

Please try to answer ALL of those questions clearly. That will help me to help you

 

slrphoto,
I think you've sent me one or two messages but I can't find them now. I just read one about making the one second time variable because it controls a motor, but I can't find it anywhere now! There's nothing from you in my private messages inbox.

Could you please post them here on this thread. Thanks and sorry about the delay!

 

OK Steve.
I'm sorry about the delay. I lost track of this project.



This design consists of two monostables (pulse generators) and is built around an NE556, which contains two NE555 timers, probably the most famous IC among hobbyists. The first, on pins 1~6 of the 556, is triggered by the pushbutton (SW1), and generates a pulse on relay K1, which drives part of your display (I don't think you gave details).

When this pulse ends, the second monostable is triggered, and it generates a shorter pulse on relay K2, which drives the motor.

The first monostable is triggered when pin 7 goes below about 1/3 of the power supply voltage, which happens when pushbutton SW1 closes. RB and CB provide filtering to prevent triggering due to interference coupled into the input.

The duration of the first pulse is approximately 1.1 x C1 x (VR1 + R1) where C1 is in farads and VR1 and R1 are in ohms. VR1 provides an adjustable resistance between zero ohms and about 100 kilohms, and R1 acts as a "stopper resistor" so there is always some resistance in that circuit, even when VR1 is at its "MIN" end. (You need a stopper resistor of at least 1k to prevent damage to U1.)

So with VR1 at "MIN", the total resistance will be 22k (R1 only). Since C1 is 82 microfarads, or 0.000082 farads, the first monostable's pulse duration will be 1.1 x 22000 x 0.000082 which is about 2 seconds. With VR1 at "MAX", the total resistance will be about 122k so the pulse duration will be about 11 seconds.

The actual maximum time may be up to 20% longer or shorter than that, because potentiometers usually have a +/- 20% tolerance on their end-to-end resistance. There is also inaccuracy in C1 and I have suggested three alternative components. The middle one is an 82 uF aluminium electrolytic with a +/- 20% tolerance, and the other two are Tantalum capacitors with tighter tolerance but different capacitance values; the time range will change in proportion to the difference in capacitance.

As well as initial value error, temperature and aging will affect the capacitance of these capacitors somewhat. Tantalum capacitors may be more stable than aluminium electrolytics. Using capacitors like this is a compromise for simplicity. It's possible to get much more accurate timing using a crystal-controlled circuit but this needs a lot more components. Let me know if you want to look into that possibility.

The first monostable's output on pin 5 goes high (+12V or somewhat lower) during the first monostable's time; this voltage activates relay K1. The relay type I've suggested has a fairly low coil current (below 20 mA); the 556 can drive up to 200 mA from its outputs.

When pin 5 returns low, the network of RS, CT and RT generates a low-going pulse on pin 8, which triggers the second monostable. The second monostable operates exactly like the first, and activates relay K2. Its timing is controlled by C2, a 15 uF capacitor, so its output pulse is about 5.5 times shorter.

I've shown each of VR1 and VR2 as a potentiometer, i.e. a user-adjustable variable resistor with a shaft for a knob. If you don't need user-accessible adjustment, you could use a preset potentiometer (aka a trimpot) for either or both.

The contacts of the relay I've suggested are rated for 3A at 250VAC and 3A at 28V DC. The motor is an inductive load and should have a diode connected backwards across it, rated for at least as much current as the motor draws. For example, a 1N4001 is rated for 1A. This is needed to protect the relay contacts from "back EMF" which is a high voltage spike that can be generated by the motor when power is removed due to its inductive nature. The two 1N914 diodes across the relay coils are there for the same reason (the relay coils are inductive).

CD is a decoupling capacitor to suppress voltage spikes due to relay switching and switching within U1.

I've shown the power input and relay output connections on a 6-pin connector, and the pushbutton just connected straight up to the circuit. I imagine you'll want to change this to suit your requirements. I'm sure you know how to deal with the mains voltages on K1 safely.

Any questions feel free to ask.

Again my apologies for the delay in getting this ready.

 

Hi Steve,

Yes, CT is 0.1 uF the same as CN, CD and CF. The leftmost three are supposed to be covered by the "3X 0.1UF" marking and CD has its value marked separately, but CT is not listed in the "suggested components" section.

You mentioned that there's no display on this project. Do you want some LEDs? I thought that the external devices (motor and whatever the other thing is) would be enough feedback for checking that the circuit is working, but LEDs can be added easily. Just add an LED with a 560 ohm series resistor across each relay coil, cathode to ground.

Sorry, I forgot to answer your question about slowing the motor down. You can slow it down to some extent by connecting diodes in series with the power source. Something like a 1N4001 will be suitable. Anode goes to the power source, cathode to the positive side of the motor. Each one will drop about 0.8V. But you will also lose torque. Obviously the best answer would be to gear the motor down.

I'm concerned about the motor. The card ejector needs some kind of mechanical reset, or feedback to tell it when to stop. If it's a continuous action, the exact duration of the motor run will be critical; if it's even slightly too long or too short, the errors will accumulate over time and it will get steadily further out of step. Has this been taken care of?

Edit: Yes, you can reduce the value of C2 to shorten the motor pulse duration. 15 uF gives the range I calculated (approximately), and the range is proportional to the C2 value, so choose the range you want and adjust C2 accordingly.

 

Yes, you can just change C2.

The suggested value, 15 uF (microfarads), gives a range of roughly 0.4~2.0 seconds; the timing range is proportional to the value. So if you change C2 to, for example, 2.2 uF, the timing range will change by factor of 15/2.2 as well, which will be 0.06~0.3 seconds.

 

A resistance of "2K2" is 2.2 kilohms, which is 2200 ohms. The "k" takes the place of the decimal point.

R1 and R2 are the stopper resistors for the two timing circuits. You can change the values of R1 and VR1, and R2 and VR2, to vary the adjustment range, but don't reduce R1 or R2 below 1k.

+motor_supply------------> <------------------ motor ----------
. . . . . . . . . . . . . relay contact . . . | . . . . . . . . . . . |
. . . . . . . . . . . . . . . . . . . . . . . . . . ---------|<|------------|
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . diode . . . 0V

The positive motor supply goes through the relay contact (in the control circuit) and the switched voltage from the other side of the relay contact goes to the motor, which is returned to 0V. The diode connects directly across the motor, with its anode to the 0V rail.