Simple LED pulser

[mawyatt]

While posting on EDN Design Ideas a reader reply mentioned using this inverted transistor to pulse a LED. Adding a LED in the inverted 2N3904 Collector to Ground produces a simple LED pulser.

Check here for the circuit concept

https://www.eevblog.com/forum/testgear/fooln-around-with-dso-awg/msg5939296/#msg5939296


And here for EDN article, note reader comments:

https://www.edn.com/unusual-2n3904-transistor-circuit/

Best

[Dan N]

Interesting idea.

When I needed an LED flasher for a piece of equipment where the supply could vary from 6-14V I found that almost all of the common flasher circuits frequency varies with voltage.  Does yours?

I found this circuit where the frequency doesn't vary much and used with R=1.5M, C=1uF, and added 1k base resistors because simulation showed peak base currents higher than I liked:
https://web.archive.org/web/20190415225416/http://www.josepino.com/light/transistor_led_flasher

[mawyatt]

Interesting idea.

When I needed an LED flasher for a piece of equipment where the supply could vary from 6-14V I found that almost all of the common flasher circuits frequency varies with voltage.  Does yours?

I found this circuit where the frequency doesn't vary much and used with R=1.5M, C=1uF, and added 1k base resistors because simulation showed peak base currents higher than I liked:
https://web.archive.org/web/20190415225416/http://www.josepino.com/light/transistor_led_flasher

Yes the pulse rate varies with bias voltage as this determines the charging current for the shunt capacitance. This circuit is about as simple as possible, requiring just 1 resistor, 1 cap, 1 transistor and the LED.

Best

[jwet]

Interesting thread.  The negative voltage stuff is just weird.

Note 555 type circuits don't vary with voltage, the charging current is an RC but the thresholds are 1/3, 2/3 of Vcc so track.  Modern CMOS 555's draw flea power.

[themadhippy]

bring back the lm3909

[jwet]

One the 3909 vector- I ran into this recently- Ken Sheriff reverse engineered one- fascinating hacking.

https://www.righto.com/2021/01/reverse-engineering-low-power-led.html