Circuit not working like intended...help please!

[BanksBunny]

Hi Guys,

I am very new to electronics and had to come with up a little project to help a friend. It's basically an IR reflective sensor that triggers a Piezo to buzz when the IR sensor detects an object. I decided to use a simple 555 Timer to control the Piezo.

I've attached the design schematic I came up. It's probably got a load of errors but with my basic electronics knowledge I thought it should function as intended, not sure what I stuffed up. As it stands, when you connect 12V to the board it simply buzzes non-stop regardless of the IR detector detecting anything.

Any help/tips/ridicule is welcome

 

[BanksBunny]

I noticed Pins 6 and 7 of the 555 Timer should be connected based on schematics on the web. Tried connecting them together with some solder but same result, buzzer just continually buzzing.

 

[Harald Kapp]

I noticed Pins 6 and 7 of the 555 Timer should be connected

... and not connected directly to Vcc (link).

Check the voltage on the Trigger input (555, pin 2). It should be high without object "in sight" and go low when the IR light from the LED is reflected onto the Phototransistor.

The LED is powered by ~ 50 mA. At this current, the CTR of the sensor is ~ 10 %, resulting in 5 mA when an object is detected. However, without an object "in sight", the relative collector current is still ~ 0.2 × Imax:


This means the dark current of the phototransistor is 5 mA × 0.2 = 1 mA. 1 mA through R3 results in a voltage drop across R3 of V(R3) = R3 × I = 10 V. This will turn on U2 via R1. Thus there will be no trigger.

What you can do:
- reduce the LED current to 10 mA ... 20 mA
- decrease R3 for a lower voltage drop. Use a potentiometer to adjust the sensitivity.

Additional note:
In addition to that, your circuit will be sensitive to any IR light from the environment. To improve on that you could:

• modulate the IR LED with an AC signal, e.g. a 10 kHz square wave (easy with another 555 used as astable multivibrator).
• use a capacitor to decouple the output (node emitter / R3) of the sensor such that only the AC component remains.
• use a diode and a capacitor to rectify and detect the modulated signal.
• use a comparator (again a 555 can be used) to generate the trigger signal from the filtered output of the rectifier.

With this method any static or slowly changing IR signal will be blocked by the capacitor. Only the reflected 10 kHz signal will be detected and trigger the monostable multivibrator.

 

[BanksBunny]

Thanks Harald,

Plenty of info there for me to go off and study some more.

Just curious, as it stands, is the way the circuit is currently behaving correct? As in, as soon as I connect power the buzzer continually buzzes. Trying to analyse the circuit to explain why this is happening as you stated the circuit won't trigger as it is so not sure why the 555's output is constantly on to drive the buzzer.

 

[Harald Kapp]

Just curious, as it stands, is the way the circuit is currently behaving correct?

Obviously not - as in correct =) as expected.

Trying to analyse the circuit to explain why this is happening as you stated the circuit won't trigger

On the contrary: it is permanently triggered.
You need to measure the signal at the trigger input. If the trigger input is held low, as I suspect (see my description above), teh output is permanently high and the buzzer is active - just what you observe.

 

[BanksBunny]

Obviously not - as in correct =) as expected.

On the contrary: it is permanently triggered.
You need to measure the signal at the trigger input. If the trigger input is held low, as I suspect (see my description above), teh output is permanently high and the buzzer is active - just what you observe.Thanks Ha

 

[BanksBunny]

Thanks Harald, you've been an immense help. Back to the drawing board it is. Maybe this time I'll post the schematic BEFORE I have PCBs made

 

[AnalogKid]

What is the piezo beeper you are using? Part number / web page / datasheet?

Specifically, does it beep continuously when powered by DC, or does it require an oscillating signal?

Is the 555 supposed to be a monostable timer, making the piezo beep for a short period of time, or is it an astable oscillator driving a piezo element?

ak

 

[crutschow]

Maybe this time I'll post the schematic BEFORE I have PCBs made

You should always breadboard a circuit before building a PCB, if possible, to make sure there are no errors and the circuit works as intended.

 

[Harald Kapp]

Specifically, does it beep continuously when powered by DC

I assumed that from the fact that the 555 is operated as a monostable mv (cf. the correction in post #2).

Maybe this time I'll post the schematic BEFORE I have PCBs made

You may still be able to salvage the pcb - without the AC optimization I proposed in post #3, of course.
For example, increase R4 to 1 k instead of 220 Ohm.
Reduce R3 from 10 k to e.g. 3.3 k or use a potentiometer, as I suggested, too. You can then "play" with the sensitiviy.

Back to the drawing board it is

That would be premature. You first need to identify the problem, find a cure and then it's time for a redesign. Use the existing pcb as a basis and don't be afraid to remove components, traces or add them as required.

And DO measure the voltage at the trigger input. Curing symptoms is not a big help if you can't identify the cause. Measurements of vital signals are an important step in that direction.