Need some design help!

Thread Starter

OkotoksModeller

Joined Mar 31, 2020
3
OK. So I'm trying to design a sensor circuit that will allow a reflective optical sensor to replace a hall effect or reed sensor.

Before I get a question asking why not just use one of the aforementioned sensors, it's because the source of the input will be a brass flywheel turning at potentially high speeds, and I don't have the tools nor mechanical know-how to mount a magnet to it and properly counterbalance it for smooth operation.

So, I would like to design a sensor circuit around the ON Semiconductor QRE1113, but need some help

There are four possible leads from the control circuit this will connect to

Mains+ (12-18 volts DC)
Vcc+ (5.1 volts DC)
SIG (5.1 volts DC)
GND (common ground)

The control circuit is designed to trigger the particular event I want when SIG is shorted to ground (pulldown?), so I need to emulate this using the optical sensor,

Can anyone help me? I'm not opposed to spending a little money here!

Thanks!
 

dl324

Joined Mar 30, 2015
10,461
Welcome to AAC!

To avoid confusion, please attach a block diagram of your intended circuit.

Sensor in question:
1585685537655.png1585685652948.png
1585685581527.png

There is an analog and digital version, which are you using. Will the flywheel reflect sufficient light? Didn't look up specs for the phototransistor, but will there be other sources of IR to affect the sensor?
 
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Marc Sugrue

Joined Jan 19, 2018
121
So are you saying you want to power the LED for the QRE1113 from, lets say the 5.1V with respect to GND and the Output transistor of the QRE1113 just has to provide a Go/No go signal?
 

Thread Starter

OkotoksModeller

Joined Mar 31, 2020
3
So are you saying you want to power the LED for the QRE1113 from, lets say the 5.1V with respect to GND and the Output transistor of the QRE1113 just has to provide a Go/No go signal?
Pretty much. I already have the dropping resistor for the LED determined, I just need to figure out how to pull down SIG (short it to ground, or at least very little restance).

Do you need to determine which direction the wheel is turning?
Nope, just need confirmation that the flywheel is turning. For this purpose, the direction it's going isn't important

Welcome to AAC!

To avoid confusion, please attach a block diagram of your intended circuit.

There is an analog and digital version, which are you using. Will the flywheel reflect sufficient light? Didn't look up specs for the phototransistor, but will there be other sources of IR to affect the sensor?
I've attached a basic diagram of what I'd hoped would work - it's not, because SIG isn't being pulled down enough. Sorry about the crappy Paint drawing I don't have my PCB design software handy!

Edit: Forgot to mention: The sensor and flywheel are in an enclosed plastic case, no other sources of IR light. The flywheel has or will have tach tape on it.
 

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Marc Sugrue

Joined Jan 19, 2018
121
Pretty much. I already have the dropping resistor for the LED determined, I just need to figure out how to pull down SIG (short it to ground, or at least very little restance).



Nope, just need confirmation that the flywheel is turning. For this purpose, the direction it's going isn't important



I've attached a basic diagram of what I'd hoped would work - it's not, because SIG isn't being pulled down enough. Sorry about the crappy Paint drawing I don't have my PCB design software handy!

Edit: Forgot to mention: The sensor and flywheel are in an enclosed plastic case, no other sources of IR light. The flywheel has or will have tach tape on it.
See the description in post #2 thats exactly what that does. A resistor into the collector of the transistor from Vcc and the collector becomes a signal that pulls low/high depending on state of Transistor. If you want a Hard on and Off cross over you may need to use a comparitor to choose a setpoint value/
 
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LesJones

Joined Jan 8, 2017
2,597
Can you actually give a value for "potentially high speeds " Here is a link to some information that might help you. If you go to the section of "Examples of sensor mounting" the next to last example uses a sensor similar to the one you have chosen on a pillar drill. This is not very fast. It's maximum speed is only about 3000 RPM The section sensor options and the input part of the schematic show an input conditioning circuit that will probably do what you want. (I say probably as I have no idea what you mean by high speed.)

Les.
 
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Thread Starter

OkotoksModeller

Joined Mar 31, 2020
3
Can you actually give a value for "potentially high speeds " Here is a link to some information that might help you. If you go to the section of "Examples of sensor mounting" the next to last example uses a sensor similar to the one you have chosen on a pillar drill. This is not very fast. It's maximum speed is only about 3000 RPM The section sensor options and the input part of the schematic show an input conditioning circuit that will probably do what you want. (I say probably as I have no idea what you mean by high speed.)

Les.
Les:

Took me a while to figure out why that link wasn't working. Missing the "L" at the beginning of the URL. With this tach circuit is the Input line negative or positive? In mine, it's positive, meaning the control circuit supplies +5.1v to SIG, and waits for this to be shorted to ground, which triggers the event. For example, if a toggle switch were wired between SIG and GND, nothing connected to Vcc, the event would be triggered each time the switch is turned "on"

Generally, the motors are rated up to about 9k-10k RPM, but in practice, being small hobby motors, they never get up to that speed when connected to their load.

Have you considered just buying a very inexpensive RPM meter for models? Like this: https://hobbyking.com/en_us/turnigy-tgy-apd02-optical-rpm-sensor.html?queryID=2f8ca9df6eb57d1db82001cdc231dd93&objectID=45914&indexName=hbk_live_magento_en_us_products

If you want to DIY, counting pulses and calculating rpm is quite basic with any microcontroller. The most important question is whether you can program and what language?
I'm actually not as concerned with RPM itself as I am with triggering the event (as sound being played) after a specific number of complete revolutions of the flywheel (the controller circuit is already built and marketed by another company - they designed it so the event is triggered when SIG is shorted to ground a certain number of times - say 1 time per pulse, or revolution, up (down?) to 1 time per 255 pulses). The intention being to synchronize the sound being played with the action on the outside of the model I am building.
 

dl324

Joined Mar 30, 2015
10,461
I've attached a basic diagram of what I'd hoped would work - it's not, because SIG isn't being pulled down enough.
A more complete description of what you want to do with SIG would be helpful.

Here's a picture showing operation of a darlington phototransistor in the configuration you want:
1585690949645.png
You need to add a resistor between the collector and Vcc for your circuit to work. LED current will determine how much the transistor turns on.
 

LesJones

Joined Jan 8, 2017
2,597
When I went back to my post #7 I also found that the link did not work. I have now fixed it. You did well spotting the missing "l" at the begining of the URL The schematic uses the photo transistor pulling the input positive when it conducts but that could be changed. The potentiometer allows you set the level of the input signal that switches the output between low and high. The 470 K resistor R18 provides a small amount of positive feedback to give a sharper switching transition. I read your post #5 after writing my post and the extra details of your problem (The fact the input to the circuit it was driving was not being pulled low enough.) made me think that more details of the input circuit of the device it is driving are required. I think the pull up resistor on it's input may be too low a value for the photo transistor to drive directly or some other reason. Also in post 5 you mentioned tachometer tape. I have never heard of this but assume it is sticky tape with black and white stripes. If the stripes are too close together compared with the size of the sensor then there may not be enough light reflected back. Also if there are many stripes per revolution the frequency of the input signal may be too high for the sensor when the motor is running at 10000 RPM If you just had one reflecting point per rev then the frequency would be 167 hz. If you had 100 stripes of the tape in one revolution then that would give a frequency of 16700 hz (16.7 Khz) which may be too high. Looking at the data sheet on the QRE1113
the phototransistor collector current would only be 0.4 mA with 20 mA through the IR LED (A 180 ohm resistor in series with the IR diode would give just over 20 mA.) Can you post the schematic of the input part of the circuit it is driving or measure the current to pull the input down to ground. (Just connect your meter set to a current range between the SIG input and ground.) I suspect a single NPN transistor or a logic level N channel mosfet or an LM393 comparator will solve the problem.

Les.
 
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