Electronics > Beginners
Sources for 3 Vdc
Audioguru:
You still did not post a schematic so I made one for you.
There is no such thing as a 4400 trimpot. Maybe you adjusted yours to 4400 ohms?
The trimpot needs a resistor in series to prevent the transistor from burning out if the trimpot is adjusted too low.
fixit7:
I adjusted the trimpot to get the range.
I turned it till I got zero and then turned the other direction to get the 4400 ohms to get the full range.
I experimented with 420 -> 800 ohms to see what worked to maintain the speaker at full volume.
I adjusted it in testing the circuit, so I may have to buy another one. :-(
On the previous circuit that used a 555 chip, I got the circuit to work in ambient daylight by shielding the ldr with a piece of pvc tubing.
How can I determine the correct resistor or range I need?
Thanks for the circuit diagram.
Audioguru:
The trimpot and the LDR form a voltage divider. The resistance of the LDR depends on its sensitivity and on the amount of light it has. The base-emitter junction of the transistor needs about 0.65V for it to turn on and less for it to turn off.
If the LDR is 1000 ohms (1k) in bright sunlight then for it to have 0.65V across it, Ohm's Law calculates its current to be 0.65V/1k= 0.65mA.
The trimpot voltage will be about 3.0V - 0.65V= 2.35V and its current is about 0.65mA so Ohm's Law calculates it to be 2.35V/0.65mA= about 3615 ohms.
The transistor's base current adds to the resistive currents so the value of the trimpot must be less than 3615 ohms, depending on the amount of beeper current.
The battery voltage drops as it runs down so you must adjust the trimpot to also allow the circuit to function when the 3V becomes only 2V. Will the beeper work at only 2V?
fixit7:
--- Quote from: Audioguru on June 10, 2019, 09:13:45 pm ---The trimpot and the LDR form a voltage divider. The resistance of the LDR depends on its sensitivity and on the amount of light it has. The base-emitter junction of the transistor needs about 0.65V for it to turn on and less for it to turn off.
If the LDR is 1000 ohms (1k) in bright sunlight then for it to have 0.65V across it, Ohm's Law calculates its current to be 0.65V/1k= 0.65mA.
The trimpot voltage will be about 3.0V - 0.65V= 2.35V and its current is about 0.65mA so Ohm's Law calculates it to be 2.35V/0.65mA= about 3615 ohms.
The transistor's base current adds to the resistive currents so the value of the trimpot must be less than 3615 ohms, depending on the amount of beeper current.
The battery voltage drops as it runs down so you must adjust the trimpot to also allow the circuit to function when the 3V becomes only 2V. Will the beeper work at only 2V?
--- End quote ---
Ok.
I will check if the beeper will work on 2V.
I could go the route of using D-cells or 18650s and step down the voltage.
fixit7:
--- Quote from: Audioguru on June 10, 2019, 09:13:45 pm ---The trimpot and the LDR form a voltage divider. The resistance of the LDR depends on its sensitivity and on the amount of light it has. The base-emitter junction of the transistor needs about 0.65V for it to turn on and less for it to turn off.
If the LDR is 1000 ohms (1k) in bright sunlight then for it to have 0.65V across it, Ohm's Law calculates its current to be 0.65V/1k= 0.65mA.
The trimpot voltage will be about 3.0V - 0.65V= 2.35V and its current is about 0.65mA so Ohm's Law calculates it to be 2.35V/0.65mA= about 3615 ohms.
The transistor's base current adds to the resistive currents so the value of the trimpot must be less than 3615 ohms, depending on the amount of beeper current.
The battery voltage drops as it runs down so you must adjust the trimpot to also allow the circuit to function when the 3V becomes only 2V. Will the beeper work at only 2V?
--- End quote ---
While I am waiting on replacement 8550,
With a very bright light shining on LDR, ohms is around 30.
With laser around 70 ohms.
Beeper works at full volume down to 1.7 volts.
You mentioned
"The trimpot needs a resistor in series to prevent the transistor from burning out if the trimpot is adjusted too low."
How can I calculate what resistor I would need?
I can do the calculations if you give me the formula.
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