How to read a Forrest Mims schematic?

I love the forrest mims books. they are very informative, however, many of his schematics which i have attempted to assemble have failed. His schematics are the only ones i have trouble with. I have had many successful circuits with schematics from the internet or other books. For some reason though, i just cant put his together.  The circuit i am currently having trouble with is the "Dual LED Flasher" From his book "Getting Started in Electronics". There is a picture of the schematic below. Note how he explains one set of resistors and not the other. Also how do i connect R1 and C1 with Q1's collector? Same with Q2?  I have put this circuit together about 10 different ways and my results are always the same: two LEDS which stay on constantly. this is very frustrating. any advice would help. Please be very specific. it would be especially helpful if you could put it together successfully (on a breadboard, that is) and take a picture.  

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mcmahanly (author) 6 years ago
Hey everybody. What a great community. Thanks for all the help! I have finally put the schematic together in working order. The problem the whole time was a faulty breadboard! That is why it was so frustrating, i double checked all values of all components multiple times... i knew the wiring was correct but for some reason nothing worked. It was only by chance that i condensed everything down into one corner that i realized the problem. I bridged a gap in the connection on the negative bus with a jumper wire and voila! flashing lights! thanks for all your help. Glad to get that one out of the way.
You said it would be helpful if someone could put this together successfully on a breadboard and take a picture of it. So I thought, what the heck... A picture, and a video, of my implementation of this circuit are attached.

Hopefully this link will give you enough resolution to see what wire goes where:
https://www.instructables.com/file/FYR1NAGGJ27Z1WU/

Regarding the transistors, they are both 2N3906 (PNP).  I've got the left one stuck in the top columns 7,8,9, emitter, base, collector, respectively.  The right one is in the top columns 20,21,22, emitter, base, collector, respectively. 

A 20 uF electrolytic capacitor (actually two 10 uF caps) connects the collector of the left transistor to the base of the right transistor.  The (+) side of this capacitor is connected to the collector of the left transistor, through an orange-black wire, and the (-) side to the base of the right transistor, through a blue purple wire.

Another  20 uF electrolytic capacitor (actually two 10 uF caps) connects the collector of the right  transistor to the base of the left transistor.  The (+) side of this capacitor is connected to the collector of the right transistor, through an orange-black wire, and the (-) side to the base of the left transistor, through a blue purple wire.

Hope this helps.

The video is to prove it really does blink.
led-flasher-circuit.jpg
framistan6 years ago
It has to be something simple that you are overlooking. Perhaps you are trying to use 2n3904 transistors which are NPN. Schematic calls for 2n3906 which are PNP transistors. I mostly agree with orkasecurity. most likely spot you goofed is on the 2 capacitors. Are you using 22uF not point22 uF? or are you connecting them to the proper transistor using proper capacitor polarity? Look for something simple.
orksecurity6 years ago
Hard to know what you're doing wrong without examining what you've done.

You do know that C1 connects from Q2's base to Q1's collector, and C2 vice versa -- the gaps in the lines indicate that these do NOT connect to the the other transistor's base -- right? And you realize that the dots mean the wires SHOULD be electrically connected?

You've checked the polarity on the capacitors, made sure you know which lead is which on the transistors and LEDs, and checked Yet Again that all the components are correct, right?

If I put it together on a breadboard for you, it would look exactly like it does in the schematic, with wires and components replacing lines and symbols.

(I haven't rechecked the circuit design, but it looks essentially reasonable -- standard multivibrator.)
Do you get confused at the crossovers ?