SP.764, Practical Electronics Lecture 7: Flip-Flops and 555 Timer Circuit Topics: 1) Comparator Review 2) Flip Flops 3) 555 as oscillator 4) 555 as “one-shot” Comparator Review: -V V− V+ VOUT + If VOUT is: V+ > V− +V (i.e. maximum) V+ < V− −V (i.e. minimum) +V Comparators - Feedback loop is not used. - Decides if one voltage is greater than the other. - Takes analog voltages and convert them into a series of bits. - Binary representation of 4 digits give you 16 values (4-bit converter). - Circuit above is a 1-bit converter: o “0” or “1” output depending which voltage is greater than the other. Flip Flops: R-S Flip Flop R Q S Q R ≡ Reset S ≡ Set Two Values TRUE “1” Hi Voltage FALSE “0” Lo Voltage INPUTS For some circuits: We use: R S Lo Lo OUTPUTS Q Q Holds last value Hi ≡ 5V Hi ≡ +V Lo Hi Hi Lo Lo ≡ 0V Lo ≡ −V Hi Lo Lo Hi Hi Hi Not Allowed! Once can force the output Q to be “HI” by setting S to “HI”. Similarly, one can force the Q output to “LO” by resetting R to “LO”. If one drives both R and S to “HI”, there is no guarantee about the output’s state. 555 as Oscillator: V0 8 6 + 5k IN 5k + 5k - 2 7 1 R Q S Q OUT 3 Voltage @ Pin 2 & 6 ( V2−6 ) Output of Output of Output CR CS Q Q Transistor @ Pin 7 < 1 V0 3 Lo Hi Hi Lo OFF 1 V 2 V0 3 Lo Lo Stay Stay Stay Hi Lo Lo Hi ON Output 555 as Oscillator: V0 8 V0 6 + 5k RA - 5k RB + 5k - 2 7 C 1 R Q S Q OUT 3 Assume there is no charge in the capacitor at start. Because VCAP is at 0V and it connects to pins 2 and 6, the input is at 0V at time = 0. When the circuit is powered up, the capacitor starts charging. When the VCAP reaches 2/3 V0, the transistor turns on and grounds pin 7. Therefore, the capacitor starts to discharge through RB until VCAP reaches 1/3 V0, at which point the transistor turns off and the capacitor starts to charge up again. V @ pins 2&6 V0 2 V 3 0 1 V 3 0 t Transistor ON OFF t Q V0 t

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