I have modified a schematic for my project with the following image:

enter image description here

Instead of a 20K potentiometer, I use 100K for more precise adjustment.

Do I need to add a disk capacitor of 10nf or 100nf to Pin 5 IC?

Is the resistor of 1K between Pin 3 and Q1 a good choice?

Is it better to use NPN or PNP transistor?

Do you think the modified circuit is good or should I add another component?

  • \$\begingroup\$ What are the characteristics of the lamp? Replacing a relay with a transistor is not a trivial change; you need to share grounds with the load; with a relay that's not required. Also typically a relay is required for hefty loads, whereas this transistor is rated for 800 mA - this will be insufficient for switching automotive-style 12V light bulbs. \$\endgroup\$ – anrieff Apr 22 at 20:05
  • \$\begingroup\$ @anrieff I have mentioned the bulb as an example, the circuit is designed to power no more than 500mA \$\endgroup\$ – Smekeri BezPrezimena Apr 22 at 20:20
  • \$\begingroup\$ If you want a larger adjustable range , you should indeed replace the 20k potmeter by a 100k potmeter. If you want a more precise range (better fine-tuning), you could consider adding a 82k resistor in series with the 20k potmeter, or maybe even a 100k resistor in series with 1k potmeter \$\endgroup\$ – Huisman Apr 22 at 21:45
  • \$\begingroup\$ The design does not report the bulb control ld resistance and the transistor base R must be 10x bulb cold R to prevent the transistor going out of saturation and burning up. If VI x Rja the thermal resistance more than 50’C then you will burn your finger touching it. \$\endgroup\$ – Sunnyskyguy EE75 Apr 23 at 5:01

There is nothing wrong with your schematic in general PROVIDING you are within the limits of the output transistor.

The BC327 used will not support currents beyond about 600mA. From the datasheet, the DC SOA at 25degC shows the limitation.

enter image description here

The 555 is not being used as a timer in this application, simply as a comparator with a high point at 2/3*VCC and a low point of 1/3*VCC. This gives a hysteresis of 1/3*VCC.
Your change of the potentiometer to 100K Ohm is not changing this hysteresis.

If you want to change the hysteresis (ON/OFF thresholds) you could add a resistor (or potentiometer and resistor in series) between the output pin(3) and the sense point on Pins(2,6). This would adjust the sensitivity of your setpoint.

Do I need to add a disk capacitor of 10nf or 100nf to Pin 5 IC?

If the supply voltage is already stable and has output capacitance, there is little benefit of adding capacitance to pin(5). If you did add a capacitor it would not change your threshold points.

Is the resistor of 1K between Pin 3 and Q1 a good choice?

This is very device dependent. The Hfe drops significantly as the collector current increases. If you are trying to support collector currents in the 500-600mA range, then it's a good design strategy to set Ib to 1/10th Ic. This would require about a 180 Ohm resistor.
If your Collector current requirements are lower then the resistor can be increased.
With 1k Ohm, and the transistor Hfe variability the worst case Collector current may only be about 100mA.

Suggested improvements:

  1. Change the output device to a P-Chan FET, this can potentially improve the output current capability without requiring base current as the transistor does.
  2. Use a feedback resistor from either the output pin(3) or to the discharge feedback pin(7) to reduce the impact of the hysteresis.

For example, the changes might be like this:


simulate this circuit – Schematic created using CircuitLab

Note: I chose the IRF5305 because it has a large VGS capability of +/-20V. It also provides a very large current capability.

  • \$\begingroup\$ Thanks for your reply, can you set me a scheme for my project and do you is resistor of the 180R same if I put a stronger 1A transistor for example? \$\endgroup\$ – Smekeri BezPrezimena Apr 22 at 22:02

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