I've worked through Forrest Mims' Getting Started in Electronics, but cannot figure out why the below circuit (on page 92 in the fourth edition) claims that LED1 and LED2 alternate in being lit depending on the state of input IN1. From everything I know, the path via LED2 will always be at least equally attractive (i.e. of a lower or equal impedance) to the 9V current in the circuit compared with the path via Q1, so the circuit will alternate between only LED2 lit and both LED1 & LED2 lit, IMHO. I built the circuit (slightly varying the values of the resistors and using a 6V power source) which indeed had LED1 & LED2 lit at the same time when IN1 is high. Am I wrong in my understanding, or is Forrest's diagram incorrect, and if so, how to correct the circuit so it works correctly (apart from inserting a separate PNP transistor). Note that I simulated the IN1 input using a push button and a constant 5V power source.
simulate this circuit – Schematic created using CircuitLab
[UPDATE] Just for the fun of it - here is Forrest Mims' response to my notification on this issue in his book:
Thanks very much for reporting the omission you described. You are correct. All of the 100 circuits in this book were built and tested 4 times each--but not this one.
I intended to print “RED” over LEDs 1 and 3 and “GREEN” over LEDs 2 and 4 and neglected to do so.
This is copied to the publisher. For the next printing I will add the missing colors over the 4 LEDs.
Thanks again for your kind note. I believe yours is only the second omission/error found so far in this book.
Forrest M. Mims III