# How does this timer 555 and transistor circuit work?

Is this the way this circuit works ?

1. If vref = 5v on the pin #5, then Vthreshold = 5v, and Vtrigger = 2.5v

2. With Vthreshold = 5v, if voltage on pin #6 Vp6 > 5v, then Vp3 is Vo = 0

3. With Vtrigger = 2.5v, if voltage on pin #2 Vp2 < 2.5v, then Vp3 is Vo = Vcc

4. The transistor works as a current source, giving:

Ic = 15,089mA

Does the capacitor start to charge when the trigger activates in t = T/2, meaning that Vc = 0 until Vp2 < 2.5v ?

While the capacitor is charging, Vo = Vcc ?

And then when the voltage on the capacitor hits the Vc = Vthreshold = 5v, discharges and Vo = 0 ?

Does it work like this ?

NOTE: The max voltage of Vo is VCC - 1.5v, not 5v like in the picture.

• Do you really think that circuit, with those resistor values, produces a constant current of over 15 amps? I get 4 mA. Oct 26, 2020 at 0:05
• This is how i did it: VbaseQ1 = (15*5.6k)/(5.6k + 3.6k), then VbaseQ1 = 9.1304v Oct 26, 2020 at 12:36
• With V330 = 15 - 0.7 - VbaseQ, V330 = 5.1696v Oct 26, 2020 at 12:38
• If Ie = V330 / 330, Ic = Ie = 15.6654 mA Oct 26, 2020 at 12:39
• You do not state the value of Vcc in your text. The highest voltage mentioned in your text is 5 V, so I used that for Vcc. At 15 V and Vbe = 0.6 V, it comes to 16 mA. Oct 26, 2020 at 14:00