Hope we are not doing your assignment for you.
Didn't realise they still made the good old 741, horrible device by
modern standards but pretty bomb proof. Still lot's a features and
limitations to train the student rather than giving them an "ideal"
I agree with others in that I think LEDs and their current limiting
resistors should go between the op-amp output and the two supply
rails. Limitation one, the 741 can drive a max of about 20mA so
you're probably hitting the short circuit protection. I increased
the resistors to 1K (what I had easy in the box), modern LEDs don't
need 20mA drives.
Next limitation the 741 can't drive it's output rail-to-rail unlike
some modern devices, especially when you are running it at 5V, the
bottom of the spec sheet. In the real world (see attached) I measured
the output at 1.8V low, 4.3V high, and a period of approximately 6
seconds. Slow enough you can measure it with a DVM rather than a scope.
That's another of my pet hates, this is a high impedance circuit and
timing will be a bit of a race between the 47K charging resistor and
the leakage in the capacitor. Every build of the circuit will be
On to the LEDS, the one connected to the supply rail will see voltages
of 3.2V and 0.7V which should be enough swing in the right range to
turn it on and off. The LED connected to ground on the other hand
sees a low of 1.8V and won't turn off. A realy dirty trick is to add
a regular diode in series with the LED, the additional voltage drop
may be enough to extinguish the LED. On my bread board two additonal
diodes were required. Never use a trick like this in a real design
as diode forward voltages are only specified with a maximum and
possibly a typical value, never a minimum. Every build will need a
different number of diodes.
No much of a tutor not to have checked the circuit first, or is the
assignment a test of your diagnostic skills?
On Wed, Aug 18 at 01:51, 'Kim' via rLab / Reading's Hackspace wrote:
> I'm a bit new and currently studying from home. I've been trying to build a
> twin LED alternately flashing circuit with a LM741 Op Amp but having
> problems. Not helped by the fact I think there might be an error on the
> schematic I was given.