Inverted Schmitt Trigger with LDR

Thread Starter

q12x

Joined Sep 25, 2015
84
I already have this circuit and working as it is.
With the following exceptions: R1 is 1k (not 10k),in my case ; Vcc=5V ; tested with general BJT NPN transistors that i have in my collection.

Right now, the Trigger is always high and gets low when i shadow LDR.
But I want it reversed now, meaning: I want the Trigger to be always low, and when I put my hand over LDR, Trigger to get high.
I've tried to find a solution for 2 weeks, with different transistor circuits and I had no luck. Im not an engineer in electronics, so be decent with me.
Thank you.
 
Last edited:

crutschow

Joined Mar 14, 2008
24,329
It's quite simple.
Just interchange the LDR and R2.

Youi likely want R2 to be a pot (say 100k) so you can adjust the sensitivity.
 

TeeKay6

Joined Apr 20, 2019
572
I already have this circuit and working as it is.
With the following exceptions: R1 is 1k (not 10k),in my case ; Vcc=5V ; tested with general BJT NPN transistors that i have in my collection.

Right now, the Trigger is always high and gets low when i shadow LDR.
But I want it reversed now, meaning: I want the Trigger to be always low, and when I put my hand over LDR, Trigger to get high.
I've tried to find a solution for 2 weeks, with different transistor circuits and I had no luck. Im not an engineer in electronics, so be decent with me.
Thank you.
Replace R2 with the LDR. Then experiment with R1 values until you get the response you want; the best value could anywhere in the range of 1K to 1Meg, depending on the LDR you are using and how much light is available.
 

Thread Starter

q12x

Joined Sep 25, 2015
84
This is the circuit I have so far:
inverted Schmitt-Trigger with transistors.jpg
Again, I'm not an engineer in electronics, so be decent with me.
The Relay is there for the sound it makes, every time I interrupt the beam of light from LDR.
So the relay will make 2 clicks per 1 swing of hand over LDR. R7 is little and it gets quite warm, but it helps Relay to function properly. Relay is at 5V.
The Trigger will action over pin3 on IC HCF4027B. So it's in use already and i need it to be high and not drained from other components.
I have already changed the relay with a little Buzzer at 5V, but he is very silent when it ticks.
1581843509780.png
Not loud enough. (Or maybe mine is busted?) It works, but not that loud as i want it.
Any other ideas for a 'click' sound from something else?
 

Audioguru again

Joined Oct 21, 2019
872
If you can't hear a piezo beeper then you must be old like me (74) and have (normal for oldies) high frequency hearing loss.
Without my hearing aids I cannot hear a smoke detector beeping.
 

jbeng

Joined Sep 10, 2006
71
Here are some mods I would probably make to your circuit.

a) move the relay so Q3 is performing what is called "low side switching", instead of "high side"
b) add a diode across the relay as shown to avoid damaging Q3 due to reverse EMF (aka inductive kickback) when the relay switches off
c) R7 is getting hot because when light shines on the LDR, 50mA is being pulled though it (via Q2 being turned on). That's 5v/100=50mA & 5v x 50mA = 250mW (1/4 watt). Change it's value to 2.2k and use it as a base resistor as shown instead.

See if these changes make any difference.

inverted Schmitt-Trigger with transistors.jpg
 
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