555 timer to turn on 2nd 555 timer

SeniorCitizen · Apr 23, 2017

Hi,
In the diag below I have 555 Timer A which turns on LED in 6 seconds and I have 555 Timer B that is basically what I need for a water rocket parachute deploy release. (started this new hobby with Grandkids). Both circuits work fine individually. My question is how do I "connect" the two so that A turns on B (in this example I would want B to turn on after the 6 seconds). Being new to circuitry I tried to connect various pins from one to the other--not doable. After googling I think this is possible but nowhere can I find a simple instruction on how to do this. I appreciate (and my Grandkids will too) whoever might be able to help me. Thanks, Richard

Alec_t · Apr 23, 2017

Connect pin 3 of the first 555 via a (e.g 33nF to100nF) capacitor to pin2 of the second 555. Connect that pin 2 via a ~47k resistor to the +ve supply rail. Connect a diode in parallel with the resistor (anode to pin 2, cathode to +ve rail).

SeniorCitizen · Apr 23, 2017

[MOD Note: fixed image orientation]
Note to @SeniorCitizen ... it helps people help you to have images that are easily read

SeniorCitizen · Apr 23, 2017

Alec,
Thank you for the prompt detail reply.
Below is the "modification" I did. When I power up, the A timer waits 6 seconds and then the red LED lights up, but the B circuit doesn't seem to have power (I put a green LED to verify). Could you pls let me know if I mis-interpreted your above suggestion. If my diag of the mod is OK then I'll recheck and recheck my wiring. Not sure if this makes a difference, but I breadboarded "A" to one power rail and "B" independently to the opposite side power rail. And coming out of the 48K (with parallel diode (your note above)) I connected to the "B" power rail. Thanks, Richard

Alec_t · Apr 24, 2017

Your interpretation of my instructions is correct; however, without having seen the schematic for B my interpretation of "turn on B" may have been a wrong guess. I assumed you meant trigger B. Sorry.
Can you explain what "turn on B" means? Does B have a continuous power supply or do you want A to establish that supply? Is the B supply 5V or 9V (can't tell from the image)? Under what conditions do S1 and S2 operate?

SeniorCitizen · Apr 24, 2017

Alec,
Again....thanks a lot; here's more info:
1) please disregard the voltage on the diagrams; the battery supply would be 3.7 lipo battery)
2) initial state of A & B would be NO POWER
3) when A gets 3.7 vdc it begins to count down
4) At the end of 6 seconds A would then establish power (3.7 vdc) to the B circuit board
5) B would then immediately give a signal to the ServoMotor and the 60 degree (full travel) rotation would release the parachute.
6) B could stay in the power state until I retrieve the rocket.
7) the B circuit as-shown (which I found on-line) has two switches S1 and S2; I don't need the switches. I need to have that "switch-area" modified so that when power to B is established, the servo motor immediately turns-on and rotates the 60 degrees. While you were helping me with this A to B interface, I've been googling to find out how to do that (not sure how yet). There is another board MSP430 that does all this, but I want to use the inexpensive 555's

I hope I've answered all your questions.
Looking at my previous posts to you (I now realize) any question I post should have "full" details....sorry.
Thank you, Richard

Alec_t · Apr 24, 2017

Hmm, that throws up a couple of problems:
1) the bipolar version of the 555 is not specified for a supply voltage less than 5V and
2) if you used a CMOS version instead it might not have the drive capability for a servo. Are you sure your servo will run from 3.7V and its control signal input draws less than a mA or two?
If it were me I'd probably ditch the 555s and use a CD40106 to provide both the initial delay and the subsequent servo signal pulses.

Alec_t · Apr 24, 2017

This would be my approach. U1 is a (cheap) CMOS hex Schmitt inverter package. Inverter U1a provides the delay from switch-on (adjustable from ~2S to >10S with the components shown). The other inverters are strapped in parallel to boost the drive current available for the servo signal and form an oscillator which is gated on by D1. The osclllation period is ~20mS and the pulse width is adjustable from ~1mS to ~1.8mS with the components shown.

SeniorCitizen · Apr 24, 2017

Hi,
Thanks for the heads-up for a better approach. I will look into buying the CD40106.
To answer your question, yes the servo runs fine on the 3.7vdc (CW & CCW with enough poop to release a rubber band). I ran circuit B many times by itself. With that in mind could you let me know your suggested modification.
I appreciate your time.
Richard

Alec_t · Apr 25, 2017

Ok. Assuming you can get a 555 circuit to work satisfactorily on the 3.7V supply and that your servo is happy with that, here is the suggested modification to Circuit B. Diode D2 keeps the oscillator 'off' until the Circuit A delay ends. The trimmer allows adjustment of the servo pulse width over a ~1mS to 2mS range.

SeniorCitizen · Apr 28, 2017

Alec,
Had a few issues with the 555's incorporating the latest suggestion. However.....reason for the delay to get back to you.....that night I ordered CD40106BE as you suggested (preferred for this application). Anyway....I received it today (that was really fast) and was excited to set it up. Please see attached new modification. I hope I used the right terminals and that my "substitutions" for C2,C3 & pulse-width 10k pot are ok. (FYI, I checked battery voltage first (3.9v) and then tried combinations of both delay pot and pulse width pot. )
My latest observation/concerns:
1) The servo doesn't appear to move at all. (shouldn't it be moving CW or CCW after the delay ?)
2) Not sure if the delay part is working....since voltage (pin 8 to grd) appears immediately
3) What voltage tests can I do to check things out
4) The red light never comes on...I assumed it would light up after the delay
I've taken the breadboard apart 2 or 3 times....and the latest one, I made things very neat and spread out so I could trouble shoot....but I'm at a loss at the moment.
Thanks,
Richard

SeniorCitizen · Apr 28, 2017

FYI, here's my breadboard (without power and servo wires connected for ease of viewing).

Alec_t · Apr 28, 2017

To make up 220n you need to put your 100n, 100n, 20n in parallel (not series). As presently arranged, the combined capacitance is <20n, so the pulse timing is way off and the servo won't respond correctly (if at all).
Pin 8 is intended to be held low for the duration of the delay, so 0.7V as soon as the battery is switched on is appropriate.
One thing I forgot: a 1meg resistor aross C1 is advisable to make sure C1 discharges reliably when the battery is disconnected.

Alec_t · Apr 28, 2017

Driving a red LED, as well as the servo control signal input, from the paralleled CMOS outputs may pull the pin 8 voltage too low for reliable servo operation. You could try driving that LED from pin 2 instead, but this would need the LED plus current-limiting resistor to be connected between pin 2 and the +ve rail (not ground).

SeniorCitizen · Apr 29, 2017

Alec_t said:

Driving a red LED, as well as the servo control signal input, from the paralleled CMOS outputs may pull the pin 8 voltage too low for reliable servo operation. You could try driving that LED from pin 2 instead, but this would need the LED plus current-limiting resistor to be connected between pin 2 and the +ve rail (not ground).
Click to expand...

Thanks a MILLION Alec ! ! (redoing the capacitors in parallel did the trick). Using the cd40106 I am now able to have a delay, then rotate the servo...yippee.
One minor item... When the delay sends a signal to the servo, it rotates CW...which is fine. Then when I re-did it, the servo didn't rotate....I think because the servo was already in the full CW position. So I assumed I have to reset the servo back to the full CCW position before the next use. However I was not sure how to do that. With trial & error I figured out a "clumsy way" to reset it CCW.... by powering the circuit on/off multiple times quickly (before the red light came on) the servo moved a small amount CCW each on/off...so I did the on/off's until the servo was in the full CCW position. Then on restart...at the end of the delay, the red light comes on and the servo rotated fully CW. Is there a simple way to "reset the servo back CCW" before the next use.....some sort of a reset button thing. For example, when I retrieve the rocket, I could press a reset button, then shut the power off....ie. ready for the next use.

SeniorCitizen · Apr 29, 2017

PS: the red light and servo ( both on the same pin as I originally had it) worked fine, so I didn't have to redo the red light source (however, thanks for the suggestion...it may come handy in the future).

Alec_t · Apr 30, 2017

SeniorCitizen said:

I am now able to have a delay, then rotate the servo...yippee.
Click to expand...

Good to know.
I'll have a think re the CCW reset. Watch this space.

SeniorCitizen · Apr 30, 2017

Update to my "clumsy" approach. I put a switch between the servo and pin 8. So after the servo goes to CW, I then turn that switch OFF.....then I do the power on/off's towards CCW "without having to rush". When done going CCW, I then turn Power OFF and then turn that switch back to ON....so it's ready for the next use. It's still a clumsy for sure. Relative to your comment above, I look forward to your next reply.

Alec_t · Apr 30, 2017

Here's a modification of the circuit.

It now works in a somewhat different way.
At power-up Q1 turns on and the oscillator starts immediately, generating narrow pulses to drive the servo fully CCW. After the delay, Q1 turns off, so the oscillator generates wide pulses to drive the servo fully CW (or to some adjustable set point).
So, if you switch on briefly, and switch off again before the time delay expires, the servo should reset to the CCW position.
I've shown the switch as a simple SPST, but it might be better to use a SPDT instead so that the NC contact can be used to short R1 to ground. This would speed up the discharge of C1 (rather than rely on a slow discharge via R2).

Alec_t · May 1, 2017

Here's an improved circuit, to cater for the fact that not all gates within the CD40106 IC may have exactly the same trip threshold voltages (due to manufacturing tolerances, or if there were a severe temperature gradient across the IC chip).

Note the different connections of the gates U1b to U1f.
I've also shown LED indicators (LED1 for 'Power on' and LED2 for 'Delay expired').
D1 has been added to allow C1 to to discharge rapidly (via LED1) without the need for a DPDT switch.