Hi, I have designed an arm for a company to extend and retract a GPS antennae, the arm works perfectly, and all prototype testing has succeeded and they're into production, however the control circuit is the issue and several iterations of the circuit, and several off-the-shelf products have been tested and nothing really meets the end-user requirements, so i've had a go at making my own control circuit.

Basically, the arm uses a linear actuator, which runs on 12VDC NOM. (actually fluctuating between 13 and 17VDC). the Actuator has built in limit switches, but the end user requirement is that the power to the unit is only active during operation. which is where the trouble started. I had several options, and initially I tried a latching relay setup, but it didn't really work (for various reasons).

we are tring everything possible to avoid moving to a micro-controller, and after countless hours I am positive that system will work with 555 timers (I've tested with a 556 to get timing both ways, but two 555 timers would achieve the same result). the idea is, the remote will trigger the timer to output power for so many seconds (long enough for the arm to fully extend), then cut out all power. The limit switch will cut power at the end of the stroke regardless. then when needed the other remote button will trigger it to retract for so many seconds then cut out all power.

i've attached the latest version of the schematic (I lost all previous versions due to a computer failure, but I think this is close to the final version).

The trouble i'm having is: as soon as power is applied to the unit, both outputs are triggered, creating a direct short in the H-bridge (the H-bridge is not actually connected at this stage, but this is a direct copy of a known-working H-bridge). I've tried all solutions in this thread:
https://forum.allaboutcircuits.com/threads/555-timer-getting-triggered-on-power-up.158538/ and none have worked.

keep in mind that all of the components were chosen just because I had them lying around. once I get this working properly, I will obviously replace these components with more suitable components for manufacture.

the end user requires 50 units as soon as a fully working prototype is available, then an unspecified number of units per month on going.

if anyone has any input, or any better recommendations, I'm all ears. I am aware that everything can easily be done with a micro-controller, but I want to avoid that at all costs, and make the whole thing as passive and simple as possible. with the idea that, if a 555 fails, it can be replaced in the field with just a soldering iron (not that you would, but it's desirable to have this option).

The output doesn't have to be a mosfet H-bridge, and can be relays, the end user prefers relays, I just like Mosfets because Relays fail under vibrations and dust more often than Mosfets.

I played around with the idea of a bistable 555 setup and another 555 as a timer but I couldn't get it to work.

as it is, this version works perfectly AFTER the initial power-up triggering has timed out. Note that I added a 3-pin header for a jumper on the reset pin to try and have one circuits triggering, automatically reset the other curcuit, but it didn't really work.

Any help is appreciated.
Jason

 

Welcome to AAC!

Why do you need a monostable pulse?
The limit switch would end the signal, would it not?

 

Welcome to AAC!

Why do you need a monostable pulse?
The limit switch would end the signal, would it not?

Correct, the Limit switch would end the signal, however, there are two requirements:
1. in the event the limit switch fails (highly likely), there is a backup safety function to avoid constant power to the motor, risking fire or electrical damage.
2. if the wires are damaged by impact, etc., there is reduced risk of fire or electrical hazard (until the unit is activated again, at least)

so this whole idea is purely as a safety function. otherwise, a simple switch would be used. also the remote location these devices are used in, a switch is not ideal, they are used in unmanned equipment, operated remotely. the current configuration has a fixed antennae, however it gets damaged during a part of the operation, so they want a way to retract it during that part of operation, then extend it again afterwards to avoid damage. The unit is subject to heavy vibration and dust, as well as debris from rocks, etc. limit switches tend to not last long in this environment, neither do relays, hence my idea to use a solid-state H-bridge. although, the end user is happy using relays, it's only my idea to use Mosfets for reliability. the unit only draws between 1.5 and 3 amps (average of around 1.8A during testing), so I think Mosfets are fine without the need for heatsinks.

 

Have you tried connecting the 555 timing capacitor (C1 below) to V+ instead of ground (note capacitor polarity)?
The capacitor then generates a positive voltage at the THRS pin when power is applied, which immediately switches the 555 to the off state.
After that, the 555 will operate normally.

LTspice circuit and sim shown:

 

[/QUOTE]

Have you tried connecting the 555 timing capacitor (C1 below) to V+ instead of ground (note capacitor polarity)?
The capacitor then generates a positive voltage at the THRS pin when power is applied, which immediately switches the 555 to the off state.
After that, the 555 will operate normally.

LTspice circuit and sim shown:

View attachment 302592

Thank you Zapper, I will give it a try, hopefully this will be a simple fix, do you have any opinion on the rest of the circuit? or any better way to achieve the required result?

 

Have you tried connecting the 555 timing capacitor (C1 below) to V+ instead of ground (note capacitor polarity)?
The capacitor then generates a positive voltage at the THRS pin when power is applied, which immediately switches the 555 to the off state.
After that, the 555 will operate normally.

LTspice circuit and sim shown:

View attachment 302592

Hi Zapper, It worked in some respects. It worked in the sense where it no long send both outputs active on powerup, but now it no longer works as a timer, in that the outputs are now just basically momentary-on outputs, as soon as I release the button, they stop. I'll sleep on it now and have a think about how I can combine the two situations.

 

now it no longer works as a timer

What is "it"? Crutschow's circuit should still work as a monostable timer.

 

now it no longer works as a timer, in that the outputs are now just basically momentary-on outputs, as soon as I release the button, they stop.

Then something is wired incorrectly.
V+ is an AC ground so the timer will work the same whether the capacitor is connected to ground or V+, as the simulation showed.

Did you use the correct polarity for the capacitor, as I noted?
If the polarity is incorrect, then the capacitor will leak, causing THRS to rapidly go high and the output to thus go low.

 

do you have any opinion on the rest of the circuit? or any better way to achieve the required result?

The 555 circuit will work, if the actuator can tolerate a stall for a short period of time with the power applied.

A better approach might be to monitor the motor current with a small series shunt resistor, and shut off the current when higher than normal operating current is detected, as the current likely significantly increases when the motor stalls at the end of its travel.
If interested in exploring that, I can give you more details.

 

Have you tried a power-on RC reset on pin-4 of the 555-timer IC?
Try 22kΩ pullup to Vcc and 10μF to GND.

 

A common error by circuit designers using a 555 timer with a DC coupled trigger input is that if the trigger (pin 2) is held low longer than the timed period, the output will remain high until the trigger goes high.

In this design, if the operator were to press the switch for an extended period, the limit switches would come into play – but it is better that these are only relied on due to a circuit fault, rather than operator error.

The circuit below from my 555 timer cookbook shows how a DC coupled signal can be converted to avoid this issue, although personally I’d use a 0.022µF capacitor for C1 so that a slower falling trigger signal causes the timer to trigger.

 

A common error by circuit designers using a 555 timer with a DC coupled trigger input is that if the trigger (pin 2) is held low longer than the timed period, the output will remain high until the trigger goes high.

That's not the problem in this application.

 

Then something is wired incorrectly.
V+ is an AC ground so the timer will work the same whether the capacitor is connected to ground or V+, as the simulation showed.

Did you use the correct polarity for the capacitor, as I noted?
If the polarity is incorrect, then the capacitor will leak, causing THRS to rapidly go high and the output to thus go low.

I will try it again, but I basically removed the capacitor to ground, turned it around (polarity wise) and connected the other end to V+ and monitored the output pins on power cycling. then after a few iterations of cycling power on and off to make sure, I think worked the triggers individually and monitored the outputs, I found that as soon as I let me finger off the button the outputs stopped.

I will start from scratch again today some time and redo every wire to make sure I haven't altered anything accidentally while trying something else.

 

The 555 circuit will work, if the actuator can tolerate a stall for a short period of time with the power applied.

A better approach might be to monitor the motor current with a small series shunt resistor, and shut off the current when higher than normal operating current is detected, as the current likely significantly increases when the motor stalls at the end of its travel.
If interested in exploring that, I can give you more details.

Thanks Zapper, I actually wanted to do that from the start, I played with the idea of a comparitor circuit based on an opamp, but I just found it too messy and overly complicated (for me). maybe there's a simpler way, I'm ME not EE, so my electronics knowledge is limited to building and modding guitar amplifiers until recently, so any ideas you have I'd love to see them. I'm just chasing the most simple system possible, and I want to use the most readily available parts so that they are available from any supplier, anywhere.

 

turned it around (polarity wise) and connected the other end to V+

Which end is the "other end"?

 

Have you tried a power-on RC reset on pin-4 of the 555-timer IC?
Try 22kΩ pullup to Vcc and 10μF to GND.

Hi MrChips, Yes I tried that before posting this thread, although I didn't have a 22k resistor on hand, I soldered together a 20k and a 2k2, I found no difference. I will try it again with fresh wiring from scratch to see if I can get it to work. I have everything soldered to a piece of perfboard now which is getting really messy, so i'll start again on breadboard and see if I can get this working with all of the suggestions.

 

Which end is the "other end"?

Ha, sorry, OK, I meant that I (originally) had the Capacitor wired with the positive lead attached to the node between the Disch and Thesh pins and the Potentiometer. with the Negative side of the Capacitor going to ground. then at your advice I de-soldered the Capacitor, put the Negative side to the node, and put the Positive side to V+. Then I cycled the power a few times while monitoring. It no longer has the output active on power-up, but it is now only momentary-on while the button is pushed. one thing I did notice was the the output would rise to about 0.5V on power disconnect. this would dissipate over approximately 2-3 seconds.

 

Hi MrChips, Yes I tried that before posting this thread, although I didn't have a 22k resistor on hand, I soldered together a 20k and a 2k2, I found no difference. I will try it again with fresh wiring from scratch to see if I can get it to work. I have everything soldered to a piece of perfboard now which is getting really messy, so i'll start again on breadboard and see if I can get this working with all of the suggestions.

I will have to breadboard this for you and get back.

Here is just something to note and keep in mind when doing design and experimenting. The difference between 20k and 22k is only 10%. You would not notice the difference with one or the other. The capacitor already has a tolerance of 10%.

 

I de-soldered the Capacitor, put the Negative side to the node, and put the Positive side to V+.

That sounds correct, so I can't tell you why it doesn't work.
All that I know is that it should.

 

I will have to breadboard this for you and get back.

Here is just something to note and keep in mind when doing design and experimenting. The difference between 20k and 22k is only 10%. You would not notice the difference with one or the other. The capacitor already has a tolerance of 10%.

Thanks i'm putting together a shopping list now and i'll go to the electronics store this morning, get fresh supplies and start again from scratch with all of the suggestions. I just looked online and found they have a 555 book too which I'll purchase while I'm there. I'm hoping to have this working today with the H-Bridge in place so I can layout a PCB and get a couple printed ASAP to send away for testing. The arm is already in production with 50pcs, So I really need to get a move on with the control circuit. I tried many off-the-shelf controllers from ebay, alibaba, and the local electronics store, and there was really nothing that could meet the end user requirements. although one came very very close, but the duty cycle was too low, after 2 activations it shut down and needed to wait 15 minutes or so, before using again. I tried identifying the ship they used, but they've scratched off the top so it's unreadable. it's a completely different pinout to a 555.