I'm looking for a circuit that can receive a light signal, use that light signal to activate a relay, but only activate one relay per light signal.

Sequence example:
Light on >1s: relay one activated
Light off >1s: circuit reset
Light on >1s: relay two activated
Light off >1s: circuit reset
....

I'm looking to do 4 to 6 relays like this. I've considered going the route of using a pi zero or similar micro controller but would prefer if it can be done as a more basic circuit. Perhaps if there's an IC that has a trigger input and series of outputs. I'm open to ideas or suggestions.

 

You need a photo-transistor circuit to convert the light signal to a pulse, which can then be feed to a CD4017 IC.
It continually sequences through its ten outputs, one for each input clock pulse.
If you want them to be OFF between each ON in sequence, then you use every other output so 1 IC can do 5 relays, but the ICs can be cascaded if you want more.
There would also need to be a transistor/MOSFET buffer on each output to drive the relays.

Does it sound like that would work for you?

 

There is also a relay shift register scheme. I don't recall any details, it was in a relay logic text book way back when. OR another option would be a stepper relay. Totally non-volatile memory.

 

You need a photo-transistor circuit to convert the light signal to a pulse, which can then be feed to a CD4017 IC.
It continually sequences through its ten outputs, one for each input clock pulse.
If you want them to be OFF between each ON in sequence, then you use every other output so 1 IC can do 5 relays, but the ICs can be cascaded if you want more.
There would also need to be a transistor/MOSFET buffer on each output to drive the relays.

Does it sound like that would work for you?

That's definitely in the right direction. To elaborate the desired sequence:

Trigger on: relay 1 activated
Trigger off: relay 1 deactivated
Trigger on: relay 2 activated
Trigger off: relay 2 deactivated

and so on.

Sounds like this could do something like pair NO and NC relays to have trigger on close the circut, but that looks like it would take a trigger on/off twice for each pair. Ideally this would be a single trigger on then off for each relay. This may be an Arduino / Pi Zero application, but I thought there existed a circuit to do this.

 

So the light comes on, and is on continuously for one full second before relay 1 is activated. When the light goes off, relay 1 is deactivated immediately. Yes / No - ?

If yes, how stable is the light brightness when it is on, and how rapidly and cleanly does the light go off? The issue here is false trips.

If the light is on for only 1/2 second and then goes out, nothing happens in the circuit and the 1-second timer resets itself, waiting for the next light pulse. Yes / No - ?

ak

 

Some light sensors are much more sensitive than others, and do the choice will depend on the light intensity. So there will need to be both a light present AND a time delay past signal to operate each relay. Certainly the CD4017 counter will be able to increment that. BUT what should happen if the light is not present for the whole one second time?
It is entirely possible to use that one second delay for incrementing the counter and also operating the relay, but that will require two AND gates per relay. Not a big problem, and two input AND gates come four to a package. If you use 12 volt reed relays then the driver will be simple and the entire system can operate from a 12 volt DC source, with very low "OFF" state power consumption.
For the one second delay a good timer will be the DC4538 Dual monostable, configured in the retriggerable mode. To drive the reed relays you can use the CD4049 hex inverter, which operating at 12 volts can sink about 40 mA. And all 3 of these devices can work quite well at up to 15 volts DC. The idle current draw would be less than 1 milliamp, excepting for the light sensor, which depends on the light intensity for the selected type.
So now you have the whole design, except for the interconnections.

 

I see where I wasn't clear. The 1 second thing was arbitrary to prevent trigger bounce or false trigger. The project is working with a drone guy to make a water bottle delivery drone, and the goal is to not risk a high altitude drop or to drop all at once. The intent is a drone could fly within a few feet of ground, use the on board light module to trigger the relay module and deliver a single water bottle.

 

The system I described would be very appropriate in that the off-state current draw will be very small. So the intensity of the light source is known and repeatable. THAT makes the light sensing arrangement easier. A simple photosensor assembly clipped in place watching that on-board light. The time delay might not be required, it would be easily adjustable longer or shorter, or it could be eliminated entirely, so that the relay would operate and de-activate with each light pulse in sequence.Same scheme, just a bit simpler now.

 

The project is working with a drone guy to make a water bottle delivery drone, and the goal is to not risk a high altitude drop or to drop all at once. The intent is a drone could fly within a few feet of ground, use the on board light module to trigger the relay module and deliver a single water bottle.

I think the premise relies on characteristics of ground reflectence that are not known, and not consistent. A better solution would be an ultrasonic distance ranging module, a low cost version of an aircraft radar altimeter. These are all over ebay.

ak

 

My understanding is that the light is an operator controlled on-board light, that ground sensing is done by the operator, probably with a video link. Operator controlled lights are a standard drone feature on some models.

 

The intent is a drone could fly within a few feet of ground, use the on board light module to trigger the relay module and deliver a single water bottle.

So are you trying to use the intensity of the reflected light to determine that you are close enough to ground?

 

So are you trying to use the intensity of the reflected light to determine that you are close enough to ground?

I'm talking about using an on board light that is user activated, not ground reflection. For disclosure I was part of the initial prototyping designs for drones dropping a grenade during the counter ISIS campaign that used a separate UHF transmitter as a trigger. The finished design that I wasn't part of is essentially what you're seeing in Ukraine. I'm told the Ukraine multi drops versions are micro-controller based. As mentioned I could go that route, but the energy consumption goes way up.

MisterBill2's suggestion is looking promising so I'll dig in that direction.

 

I'm talking about using an on board light that is user activated, not ground reflection.

Is there a reason you can't just tap into that light signal for your purposes?

 

Here is a variant of MB idea in post #6.

The graph below uses 1v levels to indicate relay state: 1v=energized 0v=de-enegized.
The input pulses are longer than 1 sec.

The schematic:



The Simulation Graph:

 

The circuit in post #14 works but why use FFs when the output must switch off when the light switches off? use an AND gate with the CD4017 CLock as one input and the selected number as the second input. The 40174 might power on in the wrong state, plus it is more expensive, AND not as common.And it draws more quiet current. That matters in a drone.

 

The circuit in post #14 works but why use FFs when the output must switch off when the light switches off? use an AND gate with the CD4017 CLock as one input and the selected number as the second input.

I attempted that while designing. AND gates didn't help.

The 40174 might power on in the wrong state, plus it is more expensive, AND not as common.

The 40174 has a POR circuit to power up correctly.
Could use CD4013B's but it would be more expensive.
They are not scarce and are readily available

And it draws more quiet current. That matters in a drone.

The CD40174B only draws about 4uA max.