| Electronics > Beginners |
| transistors |
| << < (5/10) > >> |
| ledtester:
--- Quote from: queennikki1972 on August 15, 2019, 03:10:22 pm ---... Even though it calls for some odd ball #222 bulb --- End quote --- You have to bear in mind that the book was written in the '80s and the circuit was probably developed by Forrest much early than that as he has been writing about electronics since the early '70s. At that time LEDs were much more expensive, less available and much less efficient -- i.e. not as bright. Indicator lamps were implemented with incandescent or neon bulbs. In this case the circuit is designed to put out a short bright flash, and I imagine he recommends a particular bulb (which has a certain voltage and current rating) to achieve that visual effect. |
| floobydust:
--- Quote from: queennikki1972 on August 15, 2019, 12:36:39 pm ---So I tried the flashing light circuit on page 104 of Getting started in electronics. --- End quote --- Just a note, there are two resistors missing in Forrest Mim's blinky circuit- it can destroy the transistors when you crank the potentiometer to lowest resistance. There is no resistor to limit base current to Q1 when the potentiometer is cranked, and this also can damage Q2 as there is no base resistor to limit its base current either when Q1 is full on. I would add two resistors, like 470R. One in series with the potentiometer that goes to +V, and another in series with the base of the PNP. In the old days, a 222 light bulb acted as a neat flasher lamp when overvoltaged. It is a 2.25V LAMP (two AA batteries) and on 6V or 9V is very bright, for a little while. It only was rated 5hrs lifetime. |
| Zero999:
Some things to note. If you think a potentiometer is bad and don't have a replacement, simply substitute it for a fixed resistor of half the value. If the circuit calls for an incandescent lamp, but you want to use an LED. Add a resistor in parallel, as well as the usual LED and series resistor. The circuit might be designed to work properly with the low resistance of an incandescent lamp and might not reliably work with an LED. In this case it will work, but the LED might not turn fully off, especially if it's a high efficiency green, white or blue one. Finally please do a little image processing on photographed schematics. It makes them clearer, as well as saving bandwidth. |
| Jwillis:
I tried your flasher circuit and all it did was cook the transistors. So I took the liberty of redesigning the circuit to one that I tested and indeed works and it won't cook the transistor.I added notes but feel free to make any changes you like for different effects.I was to lazy to diagnose the problem with the first circuit. |
| ledtester:
For clearer images a PDF of the book is available at: http://www.wbrc.in/ham/getting-started-in-electronics-forrest-m-mims-iii.pdf |
| Navigation |
| Message Index |
| Next page |
| Previous page |